Recognizing that it is just outside the time period you are asking about let's jump to 1671 in which Nicolaes Witsen published his book Aeloude and hedendaegsche Scheepsbouw en Bestier which we translate into English as Ancient and Modern Shipbuilding and Management. If you wish - and are able to - you can read the orignal here, if not there is a book published by Texas A&M Nicolaes Witsen and Shipbuilding in the Dutch Golden Age which - in a much rearranged way with modern commentaries - gives translations to much of the original work. My answer draws primarily from this book.

Witsen in his work talks a lot about shipbuilding techniques of the time. Relevant for this question he talks about the origin of the materials as that is highly important to him as he highlights how materials from different sources have different characteristics making them better or worse to use. His commentaries on the materials needed and preferred are collected in the Texas A&M book pages 20-24.

For timber Witsen says the best material by far is oak, which he says is supplied from Westphalia and Rheinland. An unsurprising development as the Rhine river - by which this timber was simply floated down to - terminated in Netherlands and indeed shipping goods up and down the Rhine from central Germany to the coast was (and is) one of the main sources of the Dutch-Flemish economic strength. The other main timber material was pinewood, although it was much lighter and inferior to oak and as such Witsen recommends against using it for warships and ships intended for rougher seas. For lighter and less important ships and for planking in particular it was much better suited. Witsen says the very best pinewood came from the area around today's city of Kaliningrad (then Konigsberg), while the best deal (whatever that means) was in Norway. The same to areas, Muscovy and Norway were highlighted as best places to get wood for masts which had to be long and sturdy. Witsen mentions other timber kinds which could be used for different purposes on the boat (fir for yards, lime for pumps etc.) In general, it seems heavier and sturdier wood like oak or elm was recommended to be used for frames and bottom part of the ship, while lighter more fragile woods were left for the upperworks and the inside, non-critical parts of the ship.

The next material was iron and steel, for which Witsen again highlights several sources. The best seemed to be Nurnberg steel followed by Spanish (Biscayan) iron, whose qualities he praises. Both of these places are actually well known sources of quality iron from at least the middle ages. Spanish iron could be brought in by sea, while Nurnberg - although not on Rhine itself - would probably come by the Rhine similar to timber. Witsen also mentions other sources from which I would single out Swedish (and Sweden was a major iron exporter in the 17th century) however he says the iron lacks the quality of Spanish and in particular Nurnberg steel, but is 10-100 times cheaper. Witsen lists Stockholm as the worst, with Gothenburg and Danzig (not actually in Sweden, I know) as slightly better. The names are from the ports where they are acquired not actually sources of iron itself.

Another ingredient necessary for ironworks was coal, which was imported from Newcastle in England - which was major coal hub since the middle ages - with alternatives being Scotland and nearby Liege. Hemp would be produced locally, but Witsen also mentions Italy as best source of hemp, seconded by hemp from Riga and then Muscovy (Russia).

All in all, we see that Dutch imported goods from numerous places. Most important was Germany in their hinterland through the Rhine river, and then the various Baltic regions, most important which were Scandinavia and East Baltic. However, goods could be imported from other places, like England, Italy, Spain. While the source is for as later as 1671, so not exactly early Republic, the same applied for that period as the Dutch had strong presence in Baltic already by the late 16th century. Minor changes could happen, like England and Spanish imports replaced by Swedish as conflicts emerged, but the ability to source everything from numerous places gave the Dutch much flexibility in keeping their industry going