An aquarium shop will sell them and tell you how to set it up and keep it running well. That's probably the best place to get a small one (25-100 gal/day) for herps. Water stores (like Culligan) or or home improvement stores as well.
I can fill you in a bit on how they work... I make and sell RO systems... and use them at home.
The basic gist of an RO system is that it separates 'good' water from 'bad' water. There are generally four or five stages to an RO system. The first two or three stages will be sediment and carbon filters. Then there is the RO membrane which is where the 'magic' happens. The RO membrane housing has three connections, one in feed and two out feed. The two out feed connections are; 'brine' aka 'bad' or 'waste' water that the membrane rejected and there is the permeate aka 'RO' or 'good' water. The 'good' water then goes to a pressurized tank. When the tank is full the RO system detects this by pressure from the tank and then turns off the 'process'. When you open the tap to drink the RO water on you sink or misting system or refrigerator, the tank pushes the water through the final filter stage. As the tank pressure drops the system will activate again and replenish the water used.
Now there are different kinds of RO systems... not all are equal and not all will work for the same aplication. You see the RO process is slow, in general. If you need large volume, you need a larger tank and most likely a faster RO membrane. If you only drink a couple of glasses of water from the system a day, a standard tank and slower membrane are fine. There are also systems like I sell that are 'zero waste' systems. The brine aka 'waste' or 'bad' water is plumbed into the home for use later on. The concept of a zero waste system can get difficult to explain... so I don't go into it unless you want... but lets just put it this way 'it just works' lol. The reason for zero waste systems is to conserve water. An RO system will 'waste' water if it is connected to the drain. You won't even notice all the water that is wasted because you aren't seeing it. But the reality is that for every one gallon of RO made, four go down the drain. AND this is ONLY if the system is running perfect. The ratio can be as bad as 12:1. The RO process needs to have a moderate water temp and not have a super high total disolved solids rating. Pressure is also a large factor in the process. The best thing about zero waste systems is that they have an electric pump that creates the ideal pressure for the process so less water is 'wasted' back into the pipes of your home and the time it takes to replenish is optimized.
Good RO systems aren't cheap. You can find low cost systems online... but you get what you pay for. Systems in general are all kind of the same, the filters are where the money is. and there are a TON of filters to choose from. better filters before them membrane mean a longer lasting membrane... I personally have been using the same membran for almost two years I think. This is possible because I have a flush cycle on my system after each use. This keeps the membrane from clogging up and getting fouled. I worked on a system for someone (not purchased from me) who didn't have a flush valve and their membrane clogged up in 4 months... damn LA water haha.
There are also Deionized cartridges that clean the water even farther, but those aren't used in water that will be consumed by humans or animals. The taste is funny and the water is even more void of minerals.
Back tracking a bit..... the membrane is made of a fabric that only allows H2O to pass through it. The stuff that is too big to fit passes by the membrane and goes out the brine connection on the housing. This is how the membrane 'separates' the good from bad. The water that makes it through the filter is near pure... my system at home will take in water that is reading 150-170 PPM TDS and put out water that reads 7-12 PPM TDS.
If you have any questions, feel free to shoot me a PM.