Where to buy fireplace surround

I suggest you call it a fireplace screen. The surround is something else.

Try your local fireplace shops or chimney sweeps.. They should know someone who can take care of it for you.

I used to work in a hardware store, now long closed, that sold the screens seasonally. IIRC the brand we used when a custom size was required was Portland Willamette

Now - why do you need a custom size??? Most screens go in front of the opening and are made to cover a range of sizes.

The custom ones we did were scribed to the inside of openings. We would make a cardboard template, cut to fit, then trace that to a luan plywood template and then trim that up to fit. This was then used as a template to make the custom door. These were VERY custom doors and prices accordingly IIRC.