I have a resistance band, Theraband yellow, tied to rail in my kitchen. When I microwave something for 2 minutes I do 20 reps of low resistance for a warm up(keeping the resistance band short so that the force is low) and then step farther away and do 20 more reps with a higher resistance for the work set. I do external shoulder rotations for the rotator cuffs. These muscles should be conditioned for the serve to slow the arm and racket in the follow through. Lower resistance and higher reps for endurance and some strength.
I have a heavy 16" hammer in front of the TV. I do pronations & supinations 20 reps. Adjust resistance with leverage by where on the handle you hold the hammer. Don't let momentum build up with the hammer as it could stress your tendons. You can also do ulna and radial deviation exercises with the hammer. Ulna deviation strength is greater that pronation & supination strength so the hammer might have to be larger.
I bought a large gripper and in the past have used it also in front of the TV.
Stretches are important for the serve. Several can be done seated.
Very good reference on serving exercises & stretches- Complete Conditioning for Tennis
, Roetert & Ellenbecker