I assume you are talking about a cb350F as they came with discs. The cb350 twin mostly came with drums (there were a few disc bikes toward the end of the run...using 350F discs).
The only place is to drill it yourself. Get a drill press (or beg a friend) and some hardened bits. Don't forget to chamfer the ends.
There is a how to on the web somewhere, Give me a day to find it and I'll post it here.