For the longest time, HP has been one of the main competitors to TI (in terms of calculators) and they've had a nice record of supporting RPN. I'm not sure what their lineup consists of for your price range, but I'd recommend checking them out. HP's online store, Froogle, and eBay should be good places to check.
I recently bought an HP 49g+, their direct competitor to the TI-89 (MSRP is $180). There's a good amount of information on
HP's product page for the 49g+.
Good luck. RPN is fun.
