Why do you want a 20" rim for the spare? Why not get the same as the factory spare? The rim size doesn't matter, it's the overall diameter of the tire that does. For example my stock truck has 275/55R20 tires which are 31.9 inches in diameter, the spare is 275/70R17 which is 32.2 inches in diameter or a 0.3" difference which is close enough for the trucks computer. The manufacturers do this to save weight and cost. (tires for 17" rims are much cheaper than 20s)
Keep an eye out on Craigslist or visit a GM forum and check the buy/sell forums. Quite often you can find original full sets or singles for way less than new.
