The previously mentioned Harman Labs and Metro Print are the only two I've come across that advertise printing on Ilford/Harman silver gelatin paper(s) (ie monochrome only paper). Lots of other labs can do great mono prints it will be on colour media.
True monochrome silver gelatin paper may or may not produce the best monochrome prints. I can't say I was blown away by the two samples in the Metro Print sample pack I ordered. I suspect when judged unemotionally it won't justify the higher cost. But only you can make that call.