Try Orion Optical in the UK .... They did a set for me for my 10" newt upgrade , to my size spec and I think they did a good job.
They charged me :
White vixen dovetail £29
Tube rings for 315mm OD tube £54
Machined Blocks to sit between rings and vixen dovetail bar
£24
I couldn't get them made locally for under $250 (local aluminium machinist), I was also for a while looking at having a 12" ID aluminium tube rolled from 2mm aluminium sheet and tigwelded at the join too (rejected when I was offered a 300mm PVC stormwater tube offcut for nix by a local friendly plumber who had a remnant about the right length that he was going to toss anyway , he was subing at the local shopping centre were the car park was being upgraded and I saw him and asked him if he had an offcut I might buy off him .... he said I could have any offcut I wanted at the end of the day for nix , they were no use to him and were going to the tip anyway).
Not sure what the shipping was that I was charged to ship from UK to me here NSW as I also bought one of their very nice 9 p/flotation aluminium mirror cells in the same order , which I am in the process of modifying as my mirror is not standard thickness and is actially 10" and not 250mm diam , and so the sideclamps weren't quite right.
Their email address is
sales@orionoptics.co.uk
Beakon Hill in the UK also does tube ring sets to order.
I looked everywhere for quite a while and as far as I can see they are the only ones who will make tube rings to order. Other than maybe Parallax (but they are EXPENSIVE !!!!).