There's a german hotel/ tour company in San Jose, Costa Rica. I 'think' they are called 'Rough Riders'. Find them on Paseo Colon on the road to the city from the airport on RHS. Almost opposite Kawasaki dealer. Else you can find them through the tourist office on Ave Central. (that's how I did a few years ago.)
They have small DRs, maybe small XTs and sometimes a couple of 650 KLRs. 1999 prices about $200/week.
Another German who has a backpacker hotel in Manuel Antonio rents small DRs also. And there was an American trying to start up - also with small bikes - in Puerto Viejo (de Talamanca).
I wouldn't fancy my chances of renting anything in Panama, as they're not as bike crazy as they are in Costa Rica. And the roads aren't as fun.
There's also an American company that do tours on F650s and another that use Harleys, (do a web search).
[This message has been edited by t0by (edited 18 September 2002).]