Langdon is about the most reputable provider for HK slide cuts. Even better now that they offer a nitride finish, which I'd highly recommend. Cerakote is kind of not in fitting with HK quality, but just my opinion.
There are other options out there, but I know less about them personally. There are some companies that do slide cuts to direct mount various types of red dots but, those setups are generally problematic. Due to the relief cuts on the underside of all the HK hammer guns, there's not much meat for at least one of the screw holes for the optic. Even worse if attempting to direct cut for a rail type optic like an ACRO or 509T, as you will likely hit the relief cut (I had a USP9C slide destroyed by attempting that).