I use Birch Place and Adultwork. I presume those are the "usual sites" you're referring to though and in that case the answer is simply that are almost no other options, which is understandable given how rare transsexuals are and therefore how rare TS escorts are (even though a much higher % of TS's escort compared to women).
In Scotland I've also used Escort Scotland. It's not particularly reliable though but not awful. You could try Vivastreet as well but obviously that will be very unreliable. Make sure to at least to reverse image search, e.g. Google Reverse Image Search and TinEye.
You have to specifically select the TS option on AW. But given the fair few that always pop up on those other sites you mention, the proportion of trannies that escort compared to women must be ridiculously high. Like you say, their population is still tiny - but considering somewhere probably between 10-20% of ads overall on the sites you mention are TS, it shows you how many of them must do it. Then again, I suppose taking it up the arse from a stupid amount of different men somewhat goes with the territory, so if you're wanting to take hormones and grow a pair of tits anyway, why not get paid for it?
No idea what proportion of any "civilised" population would identify as Trans, but it's not so long ago I'd have argued there were more female hookers that make up British society than any sort of transgender folk.
In any event, good luck to them - and yourself finding one, if that's what floats yer boat. You certainly have plenty of options these days, as mentioned above. Or given the number that have cropped up, especially in Glasgow recently, just stick "Brazilian" into a standard search. Chances are, most of the results will have been a bloke at one stage or another.
