Unfortunately you can buy a monitor at just about any pet shop. However, this does not mean you should. Often these animals are very unhealthy, stressed and on the brink of death. They are kept in deplorable conditions, many times without a sheat source. These animals should be avoided at all costs. There are some very reputable reptile dealers as well. A good place to find a good reptile dealer is the dealer section of kingsnake.com. Another place to look is the lizard classifieds at kingsnake. Again, this is only a starting point and does not mean these places sell healthy animals. If you are dealing with a company or person for the first time I suggest you ask for pictures of the animal for sell and/or a list of references. You can also check out the The dealer Inquiry Board. Here people post their experiences with various dealers. Another very good source of monitors are reptile shows. You will be able to see the animal first hand before you buy. You can also look at the dealer/breeders other animals in order to get a sense of the overall health and quality of the dealers stock. Although it will not tell you everything it is a start. One thing to remember, the price of the animal is often a reflection of the quality of the animal. You should always consider health over price. In the end this will be less expensive.
Before you purchase a monitor you need to make sure you have fully researched the animals requirements and have a proper enclosure already set up. Some cities have laws banning or regulating the keeping large reptiles in captivity. You should check with the local law enforcement agencies in your area if any apply to the keeping of monitors.
The monitor itself should be active and alert. If an animal is "tame" or lethargic that can be a sign poor health. The monitor's eyes should be bright, clear, and opened wide. It should actively flick its tongue as it moves around. Its nose and mouth should be clear of mucus. The vent should also be clean. The base of the tail should be round and firm, there should not be any protruding bones. Healthy monitors often do not want to be held especially hatchlings. They will try to run, whip their tail, defecate, and lastly bite you. These are things healthy monitors do and if you have a problem with this do not get a monitor. Not all monitors are as feisty as this but this is what one should expect. Look under species accounts to find which species might be better suited for your needs. Often times you can find tame older monitors but make sure they are kept in good conditions with adequate temps. "Tame" monitors at a pet store often turn out to be nothing more than cold monitors and are "monsters" when given a proper set up
This depends on the species. Savannah monitors (v. exanthematicus) and Nile monitor (V. niloticus) are heavily imported and I have seen them for sale at $15. Captive Bred species command a much higher price but are worth it. They are healthier, less stressed, and should not contain parasites. Also by buying captive bred monitors you are helping alleviate the stresses on wild population. Be careful though and ask for proof (photos of eggs and the animals hatching) of the animal being captive bred as it is common practice for dealers to misrepresent their stock in order to sell it faster. Virtually all Indonesian and African monitors are imported although there are a few exceptions.
Australian monitors are all captive bred due to a ban on the exportation of Australian wildlife. The price for Australian monitors range from about $250 for yellow ackies (V. acanthurus brachyurus) to well over a thousand for the less common. However many Australian monitors belong to the subgenus odatria (dwarf monitors). This does not mean they are small by any means. The most common (V. acanthurus) can reach length up to 3', although 2' is much more common. Remember you get what you pay for.
When you purchase a monitor you should already have an enclosure set up. This includes the cage, substrate, heat lamps, any supplemental heating device, hide spots, a basking spot, water bowl and any decorations. A digital thermometer is also good to have around and one with a probe can be purchased for under $20. A good book on monitors should also be purchased if you have not already. Look under misc. for books I recommend. You may also want to purchase some leather gloves, such as welder gloves, for when you handle your monitor.
Hopefully you will not have to transport your monitors but if you do here are some methods. For smaller monitors I have used small burlap bags similar to potato sacks or pillow cases. I usually put some moss or substrate in with the monitor and tie up the end. For larger monitors it may be possible to transport them in dog carriers or other such devices. For aggressive animals you can duck tape the mouth closed to prevent the monitor from biting. THIS IS NOT FOR SHIPPING. These methods are for short trasportation such as to and from the vet etc. Remember these animals should not be left in the sun, as they have no way of escaping heat while being transported. Extreme cold should also be avoided. A small 48-hour heat pack can be placed under the monitor if it is being transported in cold conditions.