Well there is off course the qualified dentist or sleep clinic. Not only is buying from them safe but the aftercare will also be second to none. If however you are prepared to buy one yourself there are actually many places you can do so.
For example the Internet is full of websites offering MAD splints for sale. Now comes the tricky bit! Which of these websites do you actually go any buy from? Well as a general rule of thumb, if you have not tried a MAD device before, a) you do not know if it will actually work for you and b) you may find it comfortable to wear. Therefore the first thing you need to do is ensure the website offers a full money back guarantee if you are not happy with your purchase for whatever reason, you should have the privilege of sending it back. If not, why not, you should ask.
Buying from the Internet is actually very safe. Most of the MAD splints on sale look similar although due to patents will be slightly difference in appearance or function. Although the crux of a MAD splint is to move the lower jaw forward allowing air to flow more freely in the wind pipe and throat.
Something else I have noticed about a few websites I have seen on the Internet is the cost of the actual unit. Off course I have no idea of the company's overheads or outlay but the price difference on the same page on Google can differ by 600%!
An odd thing I came across is buying from aboard does not appear to be any cheaper than buying from the UK which has the added benefit of lower postal costs.
Another great place to buy is eBay. Although it is recommend only buying from sellers with good feedback. Other outlets are you local chemist and I have also seen them in health shops.