eBay is my favorite source for that, being careful on the sellers I choose of course. I usually look for professional tool brands (aka not "gun brands"), used, and preferably with some sort of calibration certification, or at least within a date range that gives you a good chance of getting something that's probably still in spec and hopefully not abused.
Seekonk is one brand I've had good success with. Preset torque drivers/wrenches/handles are a favorite. For example, here's a 9 in/lb preset "torque handle" (incidentally, works quite well for Glock front sight screws) from them for $40 used in perfect shape.
As an alternative, kind of pricey but high quality, there's the Wiha TorqueVario series screwdrivers. The #28413 is an adjustable 5-18 in/lb. Retail or MSRP is kind of ridiculous for those, but used you can find them for ~$100-130. Occasionally you can find a good deal, possibly on eBay. I think I ended up getting mine used in like new shape for ~$80.