At the risk of sounding like one of those guys that comes into a thread and goes “Doode, joo must by a GLOCK 'cuz it roxxors!”…
Glock has a 45 slimline (6 bullets in a mag, single stack). Model 36. Got a cousin who thought this thing was da bomb!
Personally, back when I felt that I needed to carry, I decided against it. For me, moving to a 40 cal (model 23) meant a significant increase in accuracy, larger clip, and the size difference was not too much, but I was using a holster. The difference in width (if I’m reading the glock website right) is ~0.05in - not really noticeable. Of course, the barrel and clip are longer on the 23.
Bond jokes aside, if I were in a situation where I thought I needed a more concealable weapon, I would like to take a look at the new Walther PPS. It’s supposed to be 1.04inch wide and available in 9 or 40. Could be worth a look.
Also seconding Phlosphr, with a twist. I got some incredibly good advice from the guys at the gun shop / range. They had a ginormous selection to try out and were really helpful. (Of course, being both a shop and a range, they wanted me to be happy enough to keep coming back for more practice / training.)