Another vote for
Undercover Tourist through the Mousesavers newsletter link!

Great service and the lowest price on the tickets we usually buy (multi-day hoppers). The Mousesavers newsletter only comes out the 15th of each month, and you have to sign up in advance to receive it. The link in the newsletter gives a few dollars off
UT's already discounted prices.
One caution, though: Disney sometimes releases promotions that require a ticket purchase (like
free dining or the current gift card promotion). Adding the dining plan to your reservation also requires a ticket purchase. Buying even a one-day ticket through Disney (to take advantage of free dining, for example) negates the savings you would get through UT or another broker. You can save that one-day ticket to upgrade for another trip, but your initial financial outlay would be more if you bought tickets through UT or another broker, then decided to book a package through Disney with a ticket purchase required.
We almost always use a room-only discount and don't get the dining plan, but you would probably want to run the costs both ways if you are staying on-site. If you are staying offsite or doing a room only on-site reservation, UT is a great place to buy tickets!