Wanted, replacement part for revell Launch Tower and Space Shuttle Kit # 4911. Part # 21. Anyone having spare parts or know where I can look would be greatly appreciated, Thanks Rich.

I took a peek at the instructions at ninfinger.org, and that part looks pretty simple to fabricate- a flat panel with some strip stock locaters at right angles. Also, a mirror image of part No. 20? I think you could make your own that would be indistinguishable from the original without too much fuss. As a very wise man once said, "You can do anything if you're willing to make three tries to get it right!"
Model railroad guys do stuff like this all the time- check out model railroad magazines for articles on scratchbuilding buildings, or even bring the mirror image part and the instructions into a model railroad store (if there's one near you) and ask for help.
Andrew