we've got quite a few rivets and various nuts & bolts, especially good stuff for VSF - they are all scale-accurate varieties from a railway accessory store in the USA (Grandtline) and they are fantastic, covering all sorts of shapes and sizes.
Here are some we used on a FOW 15mm vehicle conversion:


here for our limited range
http://www.barrule.com/Workshop/scratch%20builders%20paradise/Augmentables.html or go here for the GrandtLine full range:
http://www.grandtline.com/model_railroad/augmentables_pages.htmother types include:








The full grandtline range covers many scales and as its all railway stuff goes back to victorian era nuts&bolts etc for railway engines and carriages. They are all plastic cast and very well produced - you just cut them off the sprue and apply with a dab of glue.
For the rivets the best method i have found is to put the glue on the model, then transfer the rivet using a cocktail stick with a dab of spit on the end to pick up the rivet - when you place it the glue grabs it off the stick.
Nuts & bolts etc you can easily pick up with tweezers.
hope that helps.
jed