Shelly, if you found a Tempurpedic that you liked initially, there is a 90 day trial offer. You would have to purchase the mattress online, but you know which one you liked, so...
I've heard they are good about refunding the purchase price in those cases where people just weren't happy in the long run.
IMO it will take you about 30 days to adjust and decide, but 90 days should be way more than adequate.
http://www.tempurpedic.com/shopping-with-us/ninety-night-tryout.asp