Sometimes, if the kit is still in production, you can contact the company (look at the instructions sheet for the number or address). Let them know what pieces you need and they might sell you the sprue sheet(s) for the pieces that you're missing.
I've never had to do this, but I remember others talking about it. Worse comes to worse ... keep an eye on ebay for a started kit or a build kit or a cheap unbuilt kit.
Just my 2. HTH.
"If you're not scratch-building, you're not trying!"


-Scott
http://s491.photobucket.com/albums/rr278/modelchasm/