Wheel Spacers Question....

I was thinking on buying new rims and tires, 18x8.5 and 18x10, but I realy like the stock rims. I think I'm going to get the wheel spacers and save about $3000.00. My question: What is the perfect size to get for the back, so when you look at it from the side, it looks even with the front? Also, where would I buy them from and what company makes these?
Are the wheel spacers worth it? Any problems with them?

Thanks