That sounds like a pretty niche market. I have never come across anyone that has offered this. I would suggest doing a google search and maybe even contacting those that offer miva merchant and see whether they'd be interested in working on a deal with you to provide end user support.
IMHO, DIYHosting (or possibly any H-Sphere reseller) might be able to do it. DIY Hosting's staff is usually pretty competent about most things that come within H-Sphere and if they don't, they learn quick
Might want to drop them an e-mail and ask to be sure
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