Hi Rob,
I have owned Heralds, Spitfires and a Stag.... I had always fancied a TR4 and when I had run out of interesting things to do with the Stag decided to do something about it.
I, like you, discovered that there are rarely many for sale .... I had an empty garage for nearly a year after selling the Stag!
Things I learnt..... price depends very much what you are after, the very expensive ones usually fall into two categories; incredibly original or extensively (and properly) restored.
Good stuff can be found in the £18-22k range, but you will need to be very patient, or very lucky.
With the greatest respect to the TSSC, the classifieds don't tend to carry many examples of TR's.... the TR Register classifieds are probably a better hunting ground. (I'm a member of both)
If you are attending local meetings then word-of-mouth is your friend, put the word out and see what pops up.
Classic car weekly sometimes has some good ones..... with all adverts it is always good to watch the market for a few months to gauge which cars have been on the market for a while, that will help you gauge why that may be the case or whether a deal can be had.
good luck with the search, I found one eventually, so it can be done
...... Andy