Where to find circular OD 4-5 lenses?

Hi i just recently bought a pair of SteamPunk "welding" goggles off of Amazon as a sort of costume. I think it would be pretty cool if I could replace the original lenses with some laser safe lenses as well, preferably OD4 or greater. Only problem is that I can't seem to find anywhere that I can buy circular replacement lenses. I'm not sure what size lens I will need but when I get them tomorrow I'll be sure to report back. If anyone could help me out here I would be really grateful. :)

These are the goggle I ordered:

http://www.amazon.com/Steampunk-Vict...ilpage_o00_s00

Thanks for your help :beer: