1/4x28
There's also a metric size you can use, but I can't remember what it is... One of the gurus will jump in with that info, hopefully.
You could also weld up the holes and just start over. I wouldn't try this with the barrel in the trunnion though!
For screw sources, local hardware stores usually have specialty fasteners. If not, here's a few links:
http://www.thread-rite.com/unbrako/button.htm
http://www.fastener-express.com/subcat.asp?0=207
http://mjvail.com/holokrome/bhcs.html
http://www.midwestsocket.com/socket-head-cap-screws.htm
http://www.microfasteners.com/catalo...cts/SSCSBA.cfm
http://www.earnestmachine.com/cap_screws.htm
http://www.circlebolt.com/socket.htm
http://www.nutty.com/bsc.html