try mcmaster.com look for socket head cap screw. I am pretty sure you can get them in stainless to. Even they have button heads.

I have those same covers with 2 missing bolts. I went to McMaster to replace them and the stainless ones that long arnt cheap! Let me know if you need the measurement off my bolts to order some up. I planned on emailing one of the Ebay sellers that sell the packages and see where they get the bolts. New is almost the same price as stainless bolts for those things!

boltdepot > *

14 20 stainless steel bolts

http://www.boltdepot.com/Socket_cap_Stainless_steel_18-8_1_4-20.aspx

Thanks everyone. Does anyone know what length bolt I would need?

id imagine measure the length of the hole in your valve cover and add like 3/8-1/2" or more.  keep in mind the thickness of your gasket.  i run thick corks on my covers and they're almost 1/2" themselves. 

I've had great service with the guys at Totally Stainless for stainless hardware.

www.totallystainless.com

Thanks guys just ordered them. I'll post a pic of them once the covers are on.