Under my kitchen sink they use a 2 inch slip joint. But... I can't find any at local hardware stores, etc.
Every slip joint is 1 1/2 inch. Why is this?

Is the pipe plastic tubular, or chrome? Can you post a pic. Ive never seen a two inch slip. Other than some really old brass tub waste and overflows I found in the archives of our shop a few years back.

If PVC or ABS than use 2" x 1 1/2" bushing...
If metal, use 2" ABS or PVC Female Thread x Slip coupling and insert 2" ABS or PVC Female Thread x Slip coupling and insert 2" bushing into it from the slip end. Continue with 1 1/2" pipe.

Also, you can always use 2" bushing into it from the slip end. Continue with 1 1/2" Fernco coupling. It is Homeowner friendly and installs in seconds.

Back to you.

Milo