It's hard to find really small pins - really small is relative, of course...
The best, but not cheapest, are scale rivets. A search on eBay (check BNA model world, they have a lot of such details - they are in Australia, but shipping is decent) or such will yield some results. They come in sizes more suited to actual model building (1 mm or less, which represents a fairly large 1" head in 1/25). From BNA, you can get 100 1.2mm rivets for about 15-20 bucks, shipped. Others are more expensive.
Another option for very small pins or snaps is silver wire - I drill a hole, insert the wire and trim close to final "depth". Then paint as normal and finish the snap by sanding the paint off the end of the wire. I did that for the top on this kit:
The snaps are very small on most tops and this seems an easier way for me to apply them.
Hope that helps!