You can find pretty much any screw you can imagine at McMaster Carr. They have a super easy search engine as well. They ship super fast as well. Regular shipping will pretty much get it to you next day. To be honest though I would buy more than one box of screws if you are going to order from them since a box of 2-56 screws is like $3.00 for 100 and shipping is probably $7. They are very reasonable on shipping even on larger items. they are a pretty greater resource for stuff.
Colorado Front Range Railroad: http://www.coloradofrontrangerr.com/