The first thing to accept is that with such tiny production numbers, finding one will not be easy. The most common model, the 403, did not even number 700. Surviving examples are considerably fewer. The easiest place to find a Bristol (particularly V8 models) is the United Kingdom. Six cylinder models can, apparently, be found in Australia without too much difficulty. Bristol stopped exporting to the United States in the late 1960's and availability peters out around the Type 410. There have been a few imports since then as the 25 year federal importation blackout is permitting mid-1970's cars to be imported now.
The first place to look is Bristol Cars Limited. Bristol prepares its cars to a high standard and this probably adds value to the vehicle. Bristol tends to keep at least one used car in its showroom. Naturally, a new Bristol can be ordered to your individual requirements if you so desire.
For more affordable cars, there in one broker in the United Kingdom. Andrew Blow has a long established business and carries a number of cars in stock and the remainder for brokerage. Visits can be arranged by appointment.
The Bristol Owner's Club produces a bi-monthly Newsletter where members are free to post advertisements selling their cars or parts. There's a wanted section also.
Online, the primary source in the United States is Hemmings Motor News . There you will find the occasional Bristol or Arnolt-Bristol. You can also take a look at Exchange and Mart and Auto Trader in the United Kingdom. The Euro magazine Classic Driver has some dealer listings.
In the United States, it's also worth sending Shawn Thomas (Arnolt Registrar), Peter McGough (North American Registrar of the BOC) or myself an email as cars crop up from time to time in obscure places. Hopefully, I will be able to post details on the Board for everyone to see.
As for print media, there are usually a few cars for sale in Classic & Thoroughbred Car, Classic & Sports Car, Motorsport and Practical Classics.
The purchase or sale of a Bristol is a process that needs patience because of the thin market. There is a large price gap between quality models and "runners" because it is almost impossible to restore a Bristol for profit. Only the most desirable Zagato models, Arnolt-Bristols, Cooper-Bristols, 404's and late series 411's can command premium sale prices in excess of restoration costs. Therefore, it is import to select a model which appeals to the heart and not the wallet. The most cost effective models to run in the United States are V8 models (preferably post-410).