Here are a few things to try:
-- the Dance Directory on this site
-- accessdance.com
-- the Yellow Pages under dance studios or dance instruction
-- Google "ballroom dance" and the name of your city
Once you've found a few prospects, I'd suggest checking them out before you commit to one. Most studios have an introductory special that lets you sample a lesson or two for a reasonable price. Or you can just take a group lesson at many studios to get a feel for the atmosphere and the quality of the teaching.
Lesson pricing varies, too, as well as whether they require you to buy contracts of many lessons at a time (usually with a discount for volume). Be sure you get all the pricing info and are comfortable with the costs involved before you commit to anything. (Occasionally, a studio will put a lot of sales pressure on you. In my opinion, I'd just walk out--that's a sign of a studio that's more interested in $$$ than in dancing.)