You may be better off buying a new gun. There really are not replacement gb's; you have parts, then you have FTK's (full tune up kit).
What kind of a gun do you own and i'll try to get a link for a replacement. Until then, buying the parts to build another v.4 is going to cost around $200 - $300. If your gun cost <$200, just buy a new gun and trash your old one.
btw, welcome to the forum's. Glad to help.