You can save a few dollars and get the oem round velour instead of the pleather if you just want the rings (
found here) and there are some 3rd party companies that sell the rings, too. Just run a search on amazon/eBay for "Hifiman pad mounting rings". Also, keep in mind that the round velour and round pleather oem pads are the only ones that have easily removable rings, all of their other pad models have glued in rings, so avoid buying those if you are trying to harvest the rings.
As for dimensions, any pads with diameters from 95mm-105mm will fit well. 110mm will also work but they are liable to slip off. I have some Shure 1840 pads and they fit on the mounting ring no problem, so your 1540s should fit. I haven't ever used Shure pads on any of my Hifiman headphones so I have no idea what they will do to the sound, but there's only one way to find out! Happy listening!