First of all, congratulations on being a very wealthy man, the possessor of an income of many marks/guilden*. Perhaps you are a wealthy burgher with aspirations to more aristocratic pursuits, perhaps you are a steward to a titled master who has used your position to advance your interests and those of your children! It is likely that if you do not have a title yet, you are probably looking for one (for yourself or your heirs), because if you are wealthy enough to buy a custom armour, you are very wealthy indeed. You may not have a title, but you are -somebody-, wealthy and important in your own right. You are going places. Hell, you're there already.
However, to my knowledge (and I know more of the military aristocracy in 15th century England than in the 15th Century German lands) the simple lack of a title would not keep you from buying armour. The main barrier is the cost, and as I mentioned above, it is considerable: custom armour costs several times the price of 'off the rack' harnesses. We are talking more money than most of your economic inferiors make in several years. But that's chump change for you. So how do you go about it?
I wrote quite a lot about the customizations process here. Read that? Awesome, you have the basic steps. Find an armourer, discuss your stylistic preferences (practical and decorative), get measured, and wait for your harness.
But since you're in the German lands, what does that look like -for you-. First of all, its important to ask where you live. The armourers available to a wealthy man in Lubeck are quite different than those on offer in Westphalia and the Rhineland, and both are different than those in the famous (to us now, anyway) production centers in Bavaria and Franconia. If you are in the north and east of the German lands, you might be looking at an armour made in Lubeck or perhaps Leipzig or Wittenberg. If you are in the north-west (or are connected through Hanseatic trade routes), you may well be getting armour from Cologne, one of the great armouring centers of the Empire. This mostly produces armours of lesser quality and greater quantity for more common soldiers, but the armourers' community is large, and some fine pieces are made there. Further south, there are some armourers in Strassbourg or Frankfurt, though they are less widely famed. In the south-central German lands, the greatest armouring center (and the greatest city!) is Nuremberg. It has been producing armours by the thousand for a hundred years by now, and for about as long its products have been famous as far away as England. It is a strange city to buy armour in - only a few armourers make whole harnesses 'in house', instead armourers get together and each workshop (restricted in size by law to less than half a dozen people) specializes in one or two parts. The the South in Bavaria the armourers of Landshut are growing in fame and skill, though their best days are a generation or more ahead of them. Similarly in Augsburg Jorg Helmschmidt is plying his trade, but it is his descendant Lorenz who will make his family famous a generation later and win them the patronage of the Habsburg dynasty itself. In Innsbruck, the armourers of Muhlau are making armours of a decent quality, but Maximilian, the man who will found his Ducal armoury in Innsbruck 50 years from now, is not even a twinkle in his father Frederick's eye. Now, some other men look south of the alps to Milan for custom armours. And you are wealthy - but are you wealthy enough to travel yourself, or have yourself measured and have good measurements sent, or to send a suit of clothes? We have what appear to be custom Milanese armours made for clients in German lands, but they are for Counts and the like - not relatively wealthy men such as yourself. So you would probably get a custom harness within your more immediate vicinity, at a nearby armouring center.
Similarly the form of armour you have to choose form is not the armour that will capture the imagination of 19th century romantics, early 20th century artists and 20th century enthusiasts is several decades in the future. Your son or grandson will wear this armour in the new German fashion, with its exuberant fluting, pointed and thin silouette, short skirt and long cuisses. Of course, he will also wear his hair in long ringlets and wear a short gown that reveals his codpiece, inviting the carnal thoughts of women. There is no accounting for the fashions of younger men. Whimsical 19th century antiquarians will call this 'gothic', but for now you are no more able to order it than a man in the Truman administration would be to order a leisure suit or rhinestone-studded bell bottoms. Instead, the current fashion may still include the kastenbrust breastplate, with a 'boxy' breast and a long skirted fauld, and the great bascinet helmet. Though we think of it as a very 'German' helmet, the sallet did not become common in German lands until a decade after you are making your purchase. In general, where fluting is present it is big and bold. It will become finer in future decades, reaching 'gothic' delicacy in the 1470's and 1480's. Moreover, your armour will not yet have the metallurgical refinement achieved in later decades. According to the research of Allan Williams, German armourers didn't unlock the secrets of fully quenching and tempering their armours until the 1460's or 1470's. Before that they might use a slack quench (a weaker method of heat treatment) or not quench them at all.
In general, you are ordering an armour at a time just before the truly famous work of German armourers is made. But your kids will have some really sweet gear.
*Thalers will come later in the 15th century. Your heirs can use them for swimming laps in silver and taking money-baths.