I have an older version of a PetGear stroller which comes apart to be a carrier (which also attachs to a seatbelt in the car). This is what the newer version looks like:
We got ours when Layla was first paralyzed and we wanted to take her on more walks with us (and longer walks). We used it more than we thought we would, and she even came golfing with us! She ended up chewing on the front mesh and ripping it during car rides (she hates the car), so it didn't work so well in that sense for us, but was still very handy for carrying her places.
I think we paid around $100 for it.