The old Uncle Mike's boot grips seem to be a favorite, but are hard to find these days. You can find them from time to time on eBay.
The S&W "factory" boot grips are not expensive, at $27.75. They are made from a hard rubber. S&W reportedly used very similar Uncle Mike's grips for a while -- I can't verify that -- but now uses their own supplier and furnishes these boot grips with the small logo on many new J frames. They look good and work well, as a basic rubber grip.
Product: J Synthetic Boot Grip with S&W Logo
The ideal for boot grips for a long time was the ones made by Craig Spegel, and at $55 for black walnut, they are a bargain (prices go up for higher quality woods). I've read that Mr. Spegel invented boot grips! There reportedly is a waiting list for new ones.
Craig Spegel - Custom Pistol Grips
I recommend looking at the VZ grips made out of G-10, if you want grips that are hard but grippy. They are precision-made and fit perfectly. But I only have them (two sets) in the K frame size, so cannot say they are relieved enough on the J frame/boot grip model to allow speedloaders or moonclips to work easily.
VZ Grips | VZ Grips - Smith & Wesson J-Frame
I think the Altamonts look great, too. I had a pair once that suffered a broken screw boss, and they sent me a new pair at no cost.
Smith & Wesson J Frame (Small) Round Butt Revolver Grips | Altamont Company
There are other good companies out there making J frame boot grips, such as Badger, Eagle, etc., but I have no experience with their products.
Finally, some people report success finding inexpensive wooden grips -- many made in Thailand or the Philippines -- on eBay. I can't vouch for the fit and finish on those, and prefer "made in America" to go on my "made in America" S&W revolvers.