What vendor can provide information on a drop-in kit?

I want to put a Romeo optic on mine, but does not have cut.

Are you looking for an aftermarket slide with an optic cut to fit on an X-compact frame?

Or do you want someone to mill your current slide for a Romeo?

I think Sig sells the kits, so after-market dealer.

Norsso:

http://norsso.com/slides/sig-p320/Sub-CompactP320

Even their least aggressive slides may be too cut for some, but they're good slides and you have a variety of available optics cuts.

FWIW, Black Market Arms has been trying to position themselves as the go-to place for P320 components, including slide groups. There may be other places to check with, but it may be worthwhile to give them a call.