You can find them at just about any store, from HD, Lowe's, Target, etc.
They will vary in price, and with regard to how much you would need to do to put it up.
We have one out in our backyard, and it's up year round.....I don't think I paid more than $100 for it, though it's only 10'x10', which may be a bit small by your standards.
You may want to check into one of the screen rooms that attach to your awning, but you need to be able to bring the awning in/out, depending on weather, and that may make it difficult.
Good Luck:)