Are these easy to find in Colombia (and other SA countries as we are travelling down and then to Rio de Janiero)? Are they expensive - in other words should I buy one before I come (they are about $30 NZD here - $25 USD)

I bought one in Rio (for Bonito) and two in Quito (for the Galapagos Islands). I can't remember being shocked by the price so I suspect they were around the price you quote. They weren't too hard to find in the big cities, any large photo shop will have them or know where to get them.

I recently bought an underwater camera in a housing, waterproof up to 30m, for about US$20. So, it is not even a disposable camera. You can take the camera out of the housing and put a normal film in it. It's not the best housing, of course, but hey, for $20 I don't care if a bit of water comes in (as long as it doesn't flood).
I bought it at a dive shop in Curaçao, so not so far from Colombia.

be advised that what you pay is what you get. I have one of these too and it cost indeed something like 25 us dollars (bought it in Hoi An in Vietnam). For underwater photo's they're actually pretty terrible. you really need a flash if you use it for diving and even for snorkeling (with enough light from above) But besides you need a Flash, the pictures just came out pretty bad most of the time. very blurry and after a few times use and developments I quitted because it wasn't worth it for me. looking for a underwater housing for my digicam now, much better ;-)
good luck anyway and enjoy ;-)
Have you already left? If not, I would also suggest checking eBay for the Nikon Action Touch. It's a camera from the 80s, waterproof and not in a housing. It has multiple focal distances (unlike a disposable) and flash (although flash on a P&S is awful unless you're in crystal-clear water). It should cost $50 or so on eBay.
It's heavy, but I like it much more than a disposable, for basically the cost of 2. I wouldn't spend more because I don't do this often.
THIS is a terrible scan, but even so it's better than the results I've gotten from disposables.