I'm not recommending any particular flight school, I simply liked the way they show the pre-reqs for each add on so I dont have to type as much. Sporty's aviation offers the Atp ctp course
Sporty's Offers ATP-CTP - Sporty's Academy
You need ten hours commercial prep in a retract airplane so I would get the commercial mel first (preferably with instrument privileges if youre up to it)then the sel can be added on in a cheaper fixed gear airplane. Youll need the multi airplane time anyway! I believe a portion of the atp ctp sim time counts toward the fifty hours required for the ATP. Buy microsoft flight sim and a CH yoke and start practicing! I now fly jets for past 25 years but I can still remember the instructor yelling at me during an engine failure " mixtures forward, props forward, throttles forward, identify, verify, feather , secure"