What a fun question! It's actually kind of fascinating how little attention overlander diaries pay to the acquisition of the wagons themselves--especially in contrast to how many words might be lavished on the process of making and purchasing supplies.

On closer inspection, in a lot of cases the lack of interest makes sense. A large number of overlanders were farmers already, lured westward at least in part by the promise of free land to farm. This meant the average family already owned a farm wagon, perhaps more than one. Either using the family one or getting a new one would not have seemed like as much a big deal as buying hundreds and hundreds of pounds of flour and bacon.

But farm wagons on their own weren't suitable for the overland trails--they needed to become covered wagons for that. Keturah Belknap described in her journal how her family spent the winter of 1847-48 preparing for their trip that summer. Her husband made the yokes for the newly-bought oxen and tried to get them used to walking and pulling as a team. He also made the wagon bows--the wooden arches that served as a frame for the cover. For her part, Keturah sewed a year's worth of clothing for her family--and made the iconic wagon cover as well!

Of course, guides to the overland trails and newspaper articles, both often just thinly disguised advertising copy, insisted that the only way to do things was to buy a wagon new. In the early years, new wagons could go for as little as $65, but it was the gold rush more than anything that spiked prices a bit. Still, buying into a commercial wagon train, with wagon supplied, would only run you $100-$150 for a normal wagon plus the organizers' markup/company fee.

Some people bought their from a wainwright back at home, wherever "home" was. Other people heard rumors it was cheaper to buy a wagon at one of the outfitting/jumping off points (St. Louis, Independence, St. Joseph, Council Bluffs), and took coaches or steamers the first stage of the way.

You'd think it might be more expensive, even with the traditional "small towns cheaper than the big city" factor, because the last-chance stop for people to buy something could probably charge whatever people could pay rather than what they wanted to pay. However, the same last-ditch chance to buy was also a last chance to, well, ditch.

A lot of individuals and families showed up in Council Bluffs or such all eager to go--until they tried breaking in their new team of oxen, or saw how many other eager overlanders were heading out as well to compete for the best land. They sold their new or newly-modified wagons to street auctioneers. So it was possible to buy a secondhand but not used wagon. Additionally, and this is hilarious to me, you could also buy your wagon at a 19th century police auction. Yes, prospective overlanders who got drunk and rowdy or into other trouble might forfeit some or all of their possessions to local law enforcement to cover fines. It's not like town councils were in dire need of covered wagons, so, they auctioned them off for profit.

One final detail I'll note because it's not something you ever seen mentioned or depicted. Overlanders often decorated their wagons! They'd paint them in sharp, bright colors, or scrawl slogans (inspirational quotes!) on the sides. One overlander who had begun the trip as an eager child recalled getting excited over how the wagon train looked like a caravan of circus wagons!

Oh, and then there were the people who skipped the "wagon" part altogether and pushed their worldly goods to California in a wheelbarrow. But I'm not sure anyone has investigated how they acquired their wheelbarrows or their audacity.

Adding to this, how do you get spare parts if something breaks down? Were all wagons "one size fits all" to make sure you could go to any town and find what you needed?