I also carry my 642 in a pocket holster(I've used the Kramer version since 1995). If your heart is set on a small-of-the-back holster, most makers have their own version - I've handled the Galco(SOB) and Don Hume(DAH), and both would be serviceable. The Galco is heavier-gauge leather and thus more $$, Don Hume's holsters are thinner but still rigid and well made for less $$.
I'm not a big fan of SOB holsters, especially those with a near-horizontal carry, because your support-side hand cannot draw the gun from such a holster if your strong-side hand is injured or otherwise engaged. SOB holsters also tend to be uncomfortable(and "clunk" noticeably) when sitting in chairs or car seats; also, the standard caveat is that you may likely suffer spinal/lower back trauma from the gun crushing your lumbar spine if you fall on a hard surface on your back or are rear-ended in a car while sitting on your SOB.
I'd suggest you try out an inexpensive inside-the-waistband (IWB) holster instead, and wear the gun at the small of your back inside your pants. If you decide you like SOB carry, you can either change holsters or stick with the IWB-SOB carry; if not, you can still carry the IWB holster in a more conventional "kidney-area" carry, up front in "appendix-area" carry, on the side in cross-draw positon, or whatever floats your boat.
Personally, I like using the Barami Hip-Grip(
www.baramihipgrip.com) combined with a Tyler T-Grip adapter(
www.t-grips.com) for a VERY low-profile carry of my Smith 442 that is still pretty comfortable and mostly secure short of hand-to-hand grappling and wrestling.