My $0.02 - Keep your eyes open when driving around. A lot of times, you may have driven by a decent dish or two without ever having noticed it.
Go out into rural areas, off the highway and onto the dirt/gravel roads. If you are in a civic organization, church, etc. then tell people what you're looking for - I got a 7.5' free that way. If you see one on a building, go in and ask - I got a 10' free that way (except paying the county road & bridge crew $50 to remove it from the roof).
I have two 10' and one 7.5' and all were free, with the exception of the $50 removal fee. All of this was within the last couple of years, so good stuff can be had.
If you happen upon a good 7.5' dish, I'd say get it. It would let you get your feet wet and explore while looking for a bigger dish. Good luck and good hunting!