Chances are that in getting wheels back close to true you'll have ended with uneven tensions - if they weren't already.
It's the uneven tensions that cause the breakages - not overtightening. If the hub & rim are good quality it may be worth a complete rebuild. LBS round here charge £20.... + the RRP of the spokes, so it may not viable.
Suggestion - download the
wheelpro book, have a read, think about whether you fancy doing the job yourself. It's not that difficult with the step by step instructions in that book.
Cyclebasket are the cheapest I've found for stainless steel double-butted spokes.
EDIT - Cross posted with Sittingduck