Here is a great site with many brands (including some certified used saddles) for sale: http://www.horsesaddleshop.com/
They also have some useful information about western saddles on their site.
Personally, I do NOT like synthetic saddles; I don’t think they are comfortable, and they seem not to hold structure well and cause pressure points especially in the front.
For leather saddles, if you want bang for your buck, get a brand name quality saddle (I dont’ mean spend thousands, but don’t get some western saddle that you can’t verify somewhere on the saddle what brand it is, that’s usually how the cheap ones are…no name anywhere).
For a thinner TB, you’re probably looking at Semi-QH bars (comparable to probably a narrow/medium tree on an english saddle). And yes, barrel saddles are shorter typically, and would fit a shorter backed horse better. You can get them in round or square skirts, and usually even the square skirts are much shorter than say an equitation saddle.
Saddles are like women’s pants…no two fit the same, even though both may be “semi-QH bars” and 15 inch seat. They all fit differently. I have tried umpteen saddles on my QH gelding, and only one fits…my Dakota show saddle! I hate it, its not comfortable, the leather is like plastic (even though it was a $1200 saddle years ago), and i feel rediculous trail riding with so much bling, but it fits, so we use it! My Dakota barrel saddle is the same bar size (both are Full QH bars) but it doesn’t fit him
If possible, try the saddle on with someone who can help with proper fit.