The fact that our beloved 3rd (1st, and 2nd) Gens are long out of production does make finding a good holster challenging. In general, the 45-framed guns will have their own holsters, as will the 9/40-framed guns, and often times the many models will work within those two categories. Hooked vs round trigger guards will come into play, as will the railed TSW models and certain variations in dustcovers, but I have found leather to be much more accommodating than kydex.
My favorite production holsters for S&W 3rd Gens are the DeSantis leather OWB models. DeSantis uses excellent molds and offers very good functional holsters for the money. I'm not exactly sure that they still currently offer these, but I have had great success scouring eBay for them, and often find them at very reasonable prices. My Belt Slide holster for a 6906 in particular fits a wide variety of 3rd Gens - 6906, 5906, 3913, 4040PD, 915. Of course, it's well-broken in, but I find that I use it the most when carrying OWB.