The squat really is the best stretch for the squat. Even if you have some trouble hitting depth at first, you'll find after a few sessions that it gets easier. It'll really help to film your lifts so you can see if there are form issues keeping you from hitting depth. I find that I sometimes let my stance get too wide, which makes it difficult to hit depth. You don't see that kind of thing unless you get it on video.
Bill Hannon is an SSC in Rennsalaer; it would be worth the drive to do a session or two with him to get your lifts coached in-person. If you want to have an SSC regularly, you can sign up for Starting Strength Online Coaching, which is a great resource for people like you and me who do not have easy access to an in-person SSC. I use SSOC, plus I go up once a year or so to Baltimore to get in front of SSCs in-person.