"will a metal/machine shop bend a ring for me and how much would that cost?"
Suggest you ask some of the other builders where they got the ring from.
Yes, some fabricating shops can roll metal sections to a radius, but it will just vary as to cost and capabilities at different shops. For what you'd need on this kind of thing, you might be able to work with a light enough section that you could use a conduit bender or home-made jig or something of the sort to bend it yourself.
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"be careful this rando stuff is addictive and dan's the 'pusher'."