How old is your son?
My friend went to the fire station because her car seats didn't all fit in the back of her Toyota Corolla. They gave her a new convertible car seat and said it's the narrowest one on the market. Turns out, it's the same seat I have!
It's the Cosco Touriva/Regal Ride, but I think they've renamed it Scenera. My 2 1/2 year old son is about to grow out if it, but he's off the charts for height, so it should last most kids longer. It's rear-facing and forward facing.
I am having a car seat dilemma right now, too, and I'm thinking that I'll have to ask some friends and borrow their car seats to see if three of them will fit in my car. Maybe you could have the other mom bring over her seat, borrow an infant seat, and a forward facing seat and see if they fit.
I have found car seats on clearance, and I even bought one. It only saved me about $8, though, and I didn't realize this until I was installing it, but they are OLD. They've been on the shelf for so long that their expiration dates are that much closer. The seat I'd just bought wasn't going to last me 6 years, it was going to last only 5. And I didn't get enough of a discount to justify losing a whole year of use!
So I'd recommend NOT getting a clearance seat, unless you aren't going to have any more kids. If you aren't, your son will probably grow out of his seat before it expires, anyway.
If your son is too big for a convertible seat, look for a combination seat. It has a harness that you use until he's too tall for it, then you take out the harness and it's a high-backed, belt-positioning booster seat. They're not very narrow, though.