The S&W "knock-offs" of the UM Boot Grip are more than acceptable, IMO. I have left them in place on every one of my Js that came equipped with them. They are a smidge lighter in weight than the UMs (1.64oz v. 2.1oz on a jeweler's scale) but will have to be slightly relieved with an X-acto knife to allow the HKS loaders to function.
The UM boots (w/o UM markings) used to be available on the S&W website "Store" for 1/2 the price of the knock-offs. Can never have enough J-frames. Or Boot Grips.