You cannot buy them online. You can only buy them in certain states, as well. I bought ours in Georgia (where I live) and went there in April for 5 days. It was quick and easy.
Well, since the tickets were valid, we went back for a couple of days while we were in Orlando in June.
We went a total of 7 times to both parks at will... all for a grand total of $89 per person. Oh, and you don't pay a sales tax with it either. It's $89 period.
Next year, we're going to do it again.