I did mine quite a few years ago and don't remember it all perfectly, but it was when I was making up sets of hex bar carb linkage upgrade kits with heim bearings. When I stopped at a local bearing place I mentioned having to get new axle bearings and was told that if I brought one in they could match it. I took one to them and they cross-referenced it and said I could replace it with a different roller bearing that was sealed, same OD, ID and thickness. That's what I got. I can't remember the manufacturer from back then.
Replacing the regular, grease packed kind isn't all that difficult on an IRS rear. You have the option of either removing the diagonal arm or leaving it in place - It's easier to remove it and press the bearing out/in using a press. Just remember to peen the tabs out releasing the diagonal arm mounting bolt so you can screw it out to remove the arm, then peen them back when done to lock the bolt in place. To grease the bearing, either smear it in there with your fingers or use a large BBQ meat marinator syringe packed with grease and just shoot it in there. Check your Bentley's manual for the type of grease to use.