In no order: call beretta talk to Rick I think he runs their shop. Cole's in Maine seems to have a good reputation . I've had positive exp with both for repairs and communication. I know there are others. Mike
Unlike the Browning 'lifetime guarantee' that ONLY fixes what is broken - no thought to upgrades or subsequent problems which occur as a result of the repair (e.g,replace an ejector spring and then the ejector breaks under the new load) - Beretta tends to go over the entire gun and will replace that which is worn or not up to par. They have oversized parts, so as things wear they don't need to weld or tweak, they just put a slightly oversized part in and once fitted you are good to go.
They will charge for labor (not sure what a DT-10 rebuild costs), plus parts. Owing to the oversized parts this is much cheaper than you might think.
There is a gentleman on here regularly, James Flynn, who is a Beretta Gunsmith in CA. I doubt that he rebuilds them, but he might be a good source.
Next I would ask you, what is the problem with the gun. I have two DT10s. One is about 10 years old with a fair amount of use. It is a little loose when it opens, but no problems with how it works. The other, I bought used, so i don't know just how old it is, but I use it only for doubles and while I believe it is probably 10+ years old, it is still tight. I have never had any problems with either.
I am a big DT10 fan, and suggest, if you feel you are getting a good price, buy it and shoot it a while before deciding it needs to be rebuilt.
A DT-10 would probably need to see 750K+ rounds before even considering being rebuilt. Mine has close to 85K through it and it's not even broke in yet. If it weren't for the bluing wear on the trigger guard you would have no idea.
My guys in Argentina run 682s for 250K rounds before they start needing parts and adjustments.
I just had an excellent dealing with Cole, sent a Beretta 686 in for a going over and believe it or not the unsingle separated from the block Rich did the work in Fla (where he works for the winter from Maine) went through the entire gun replaced pretty much every spring and the such except the firing pins he said didn't need to be done,tig welded new lug and the barrel back to the block.,. Good honesty and great work, gun came back tight like it was brand new. Bill was $300 plus shipping can't beat that.