Local junkyards are always the cheapest source. Autozone, NAPA, and Advance Auto Parts are also popular. Check your local Craigslist, and eBay, for hard-to-find parts. AutoPartsWarehouse also sources cheap, generic parts.
Search for 1990 chevrolet truck parts on google and enter a website called lmctruck. Here you can find models and parts for them from 1988 - 1998. So just select your model than the year and another window will appear where you can choose the part that you need.
Auto parts stores . dealershipships. Online websites offer all types of parts. If you want used junk yards are a good placeto start or look in your local newpaper for someone seeling parts or a whole vehicle like yours to use for parts.