If you are only after something to fill you trem cavity you don't need to be too specific about your timber type. Get out your yellow pages, look up lumber yards in your area and go visit. The amount of timber needed to fill a trem cavity is negligible so they'd probably give you a small off-cut for next to nothing.
If you really must get some timber that matches your body I've dealt with Gilet Guitars in the past when I bought some American Alder off them for my custom project -
http://www.giletguitars.com.au/. They might have a scrap bit of timber that would do you.
