I'm looking to build a robot [...] moderately sized (18-24") and suitable for use outside. With wheels large enough (say 8-10" diameter)
You should try making some sketches (to scale) of what you want to build, then you'd see that the wheels are a bit too large for the chassis (unless you really want the chassis hanging between the wheel axles).
If you want to give sensors a good clearance, don't let the wheels reach further back or front of the chassis to shade their field of "view".
and enough torque to be used on grass.
Use knobby tires, or the torque will be converted to slip.
[...] finding drivetrain components: wheels, gears etc. has been a problem.
If you define precisely what you're looking for, I'm sure there's plenty of shops we could point you towards.
I've looked online, but I've been unable to find (or find matching) motors, gears, sprockets, wheels, etc to build something in the size range I want.
It's about the size of a large R/C car, so there should be plenty of possibilities with R/C components (although at a price).
Can anyone give me some guidance to resources for parts, or a pre-built chassis I can adapt? Or just suggestions to head me in the right direction??
If you decide to up the ante a bit, bicycle sprockets and chains are good for a heavy duty drive-train.
An R/C chassis from a gas driven model may be what you need (depends on the budget of course).
A baby stroller would be yet another possibility.
If you've got plenty of cash, McMasters can deliver just about any mechanical part, including sprockets and the like.
And let's not forget the many shops specializing in robot components and kits - some of them advertise here and a quick googling will turn up more than you care looking at.
I'd like to focus more on the sensors and navigation than on the mechanical drive.
All the more reason to build a stable and dependable chassis and drive then