Yes, I have used several vendors that are Mouse authorized, not just
UT. Currently UT offers some of the best deals. A nice thing about UT, is their refund policy. I've not needed to request a refund, but it's nice to know they have one. Another rnice thing abot UT is they send a copy of the ticket numbers with the tickets. Having a copy of the numbers is handy in case you lose the actual tickets, that is, if you keep a copy of the numbers in a different place than the actual tickets. (Once you open the packet, no refunds) If you ever lose WDW tickets, all you need to get the tickets replaced is that number. Without it though you may be out of luck.
Maple leaf Tickets is another vendor I've used in the past.
There's also a place called Kissimmee Guest services. I've never tried them, but supposedly they are also an official ticket vendor. I think you have to go to the office (in Kissimmee) to get the tickets.
Another resource to consider is
AAA if you have a membership. You can go in person to your local office. You can also go to the Orlando area office(s)- but they aren't open on weekends. I've never tried buying them in Orlando, but we drove past one last time.
Also, be aware, there are a number of places in Orlando that advertise Discount tickets, but many are really timeshare tours. I've never done one, but they don't generally have a solid reputation as being worthwhile. A few of the big companies may be legitimate and hold to their claims, but many are high pressure tours that go way over the two hours they promise.