I'm assuming you mean a Cal. 3313? If so, did STS say why they can't service it? That's odd - it's not a restricted caliber or anything, so they should be able to...
Can't give you a recommendation, but will say that whoever you choose would be familiar with the movement, the upgrades they often need, should be trained by Omega on co-axial servicing, and have a parts account. Choose the watchmaker carefully.
Cheers, Al
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