Got mine through Softie in California. Superb service and they can custom make a rig for you if you need it packed to a certain dimension.
However, being a shorta@$e, would a seatpack maybe not be a better option?
Also, it depends on what you're going to be flying in as to which pack works best. I got a seatpack because, flying a Pitts, I had more excess space vertically to the canopy than I did horizontally to the instrument panel. Softie also makes packs that are long and thin and designed for use in gliders which have more of a reclining couch than an upright seat. The chute starts at your shoulders and goes down under your butt rather than stopping at your tailbone. It may be another option for you.
At any rate, they're very well known, a small operation so you talk to experts in every instance, and they make sure they get it right. I've been very happy with mine.
Of course, since I've never had to use it, I have no idea if there's even a real parachute inside, but the pack looks good and fits comfortably.
Drop me a PM if you have any other questions. I'm by no means an expert, but I did do a bit of research which led me to Softie.
Cheers,
Pitts2112