Brownell's sells 'em. Starting about $150USD plus $57US for the bushing and go way up from there.
Might be best to make the time and the vise and bushings. They're really just big clamps with a bushing you can bolt to a bench.
Made one, years ago, in a machining night school class I did after graduating from the machinists course. Way heavier than required. Only got to play a couple hours per week, but it took about 8 to 10 hours. I think.
Mill two hunks of steel(used 3" SPS myself. Too big, but I wasn't paying for it.) about 1 foot long flat on one side. Drill and tap 4 holes in the bottom hunk and clearance holes in the top hunk for the bolts(used 5/8-11. easy to find bolts that big). Bolt together then drill and ream the hole for the bushing. Make bushings out of mild steel that are tapered for the chamber area of assorted barrels.
Whole thing is only a good idea if you plan on fiddling with more than one rifle. Making a barrel vise otherwise is kind of daft. Time wise.