The PH side mount has been out of production for a good many years. You need to get lucky in the EE, at a gunshow, or maybe Western Gun Parts.
IF someone would lend you one, you could probably get it copied by a competent machinist, but unless you have such a person amongst your friends/family, it could cost more than it's worth to you.
If you want a mount that will cover those holes and you don't mind making more, you could look at the Williams side mounts. I'm not sure, but that might do it.