I still have the original MERP dice that came with the boxed set I bought a few decades back. They are 20-sided D10's, and the only ones I've ever found. (To be honest, I haven't looked all that hard for new ones.)
These days it seems it's more common to use 2 D10's, with one of them numbered with the 10's digigt and the other the 1's digit, but I find that the D20's just seem to roll so much better.
Also, and I'm sure it's just my imagination, these dice seem to come up with the most appropriate results at the oddest of times. Once, a character used a magic ring that summoned a god to save him when a crit result stated that his spleen was punctured. The god gave him a magical, iron spleen. He went on to get the same crit result 2 more times throughout that character's existence, each time being able to "sluff off" the damage because of his iron spleen. No other character that I recall got that crit result. We all used the same dice back then.