Not many options here. Laserlyte is the only mfg I know of that had a rear sight mount laser for the PT-140 Pro, however they discontinued the RL-TM model due to lack of sales volume. They are extremely hard to find and Laserlyte's online dealer search comes up goose eggs. I did however locate one in stock online via Sears for a great price.
If you are seriously considering adding a rear sight mount laser to your PT-140, I encourage you to grab this one up before it vanishes for good. If you do buy it, Laserlyte will still honor the full mfg. warranty in the event you had any issues. I have a side mount Laserlyte on my Kel-Tec .380 and it is very accurate. Nice addition. Good luck!