As you know, a "general" mechanic is not the best place for transmission repairs, as their expertise rarely includes these extremely complex devices. As a result, you need to take the car to a shop that is specialized in transmission work.
However, since you mentioned a few "chain" operations, I also want to caution you about chain transmission shops such as AAMCO, Lee Myles, and Cottman. These chains are notorious for telling you that a new or rebuilt transmission is necessary, even if only less-extensive repairs are needed. Additionally, the workmanship at these transmission chains tends to be...not the best...and their prices tend to be high.
As a result, I strongly recommend that you ask friends, neighbors, relatives, and co-workers for recommendations on an
independent transmission shop in your area. Ideally, you want an indy shop that has been in business for at least 3 years. An independent shop that has been in business for at least 3 years is more likely to give you an honest diagnosis, good workmanship, and relatively decent prices.
And, for your more common automotive work, try to locate an independent shop in your area, rather than continuing to go to Pep Boys, or Firestone, or Sears, or other chains. Just as with transmission work, the chain-operated general mechanic's shops tend to be more expensive and less competent than independent shops.
For help in locating reliable and honest shops in your area, I will give you a link to a site with recommended mechanics. All you do is enter your zipcode and some other info, and it will give you some shops to consider:
Car Talk | Mechanics Files
Good luck!