If you are in Vancouver, check out BMV in Richmond. They specialize in Mercedes and BMW, at a fraction of dealer prices. They call themselves wholesale, but that just means that their focus is on supplying the non-dealer Mercedes specialists. They will sell to you. I can't remember exactly the cost of a regulator, but I think it was $30 or $35.

If not, any alternator and starter rebuilder (the one in Kamloops, for example) has Bosch or Hella stuff at reasonable prices. Both Bosch and Hella make a regulator for your alternator (same one as on my '89 190e, I believe). No difference in quality from the dealer item, except that you don't get the blue box with the Mercedes logo.

The small private parts suppliers can supply most consumable stuff at decent prices. It makes running a Mercedes very economical. Even stuff in a Mercedes box is sometimes cheaper than dealer price. The dealer markup is huge.