I like eating plastic!
While missing parts to represent a costly hassle, they're not a death sentence for a kit anymore. If you happen to misplace parts as often as I do, I recommend saving this to your favorites.
Gentei Kits Part Order Form
To look up part numbers there are two easy methods, either use the manual, or you can go to Dalong, find the kit you're missing parts from and place an order that way.
You can also use the missing part order service to buy individual pieces from other kits. For example if I wanted to just by the head components for the MG Force Impulse, I could look up the part numbers for all nine pieces. Unfortunately at $2 (or more) a part and with shipping, 9 parts will total over $20 shipped.
Shipping on parts can take a while too. You should anticipate waiting at least a month until your shiny new part arrives, but waiting is a whole lot better than not being able to finish your kit at all.