Who could fix a driver on a Mission 781monitor

Try to deal directly with Mission if you can. A few years ago I blew the woofer on one. The local dealer (who I did not buy them from) gave me a phone # that I beleive was in CA. I called and spoke with a great guy, he Fed Ex'd a new one over night, NO charge! (the speaker was not under warranty, tho it was bought from an auth. dealer) Good luck.

If it's a woofer, have it repaired by Bill Legall of Millersound, (215) 412-7700.

But, do the pair to keep them balanced, as the one he fixes will end up being better/stronger than the one you don't. Figure on a price of something like $40 - $65 for the pair.

TriState Loudspeaker in Aliquippa PA (just outside Pittsburgh). He's one of the best anywhere.

This is from the Mission UK site's FAQ:
"Mission endeavours to carry a comprehensive range of replacement parts for many years covering both current and discontinued models. Typically, these will include spares items such as tweeters, woofers, grilles, plinths and foot spike sets. Please contact our service desk on +44 (0)1480 452561 or e-mail to spares@mission.co.uk for prices and availability."

It would be worth an email to them to see if a driver is available or if they will tell you the model of the driver. It might be available from someplace in the U.S. like Madisound or PartsExpress.