Here's a point I don't quite understand:
The OP owns a 38 and has said he's comfortable shooting it, despite his medical condition. He says he wants to stock up on ammo for self defense.
Question for the OP: How much self defense shooting do you plan on ever doing? I should think a box of 20 would last for the rest of your life.
If you plan on stocking up on 22 ammo for general use, hunting, whatever, fine, but I definitely would not take the step down from a 38 to a 22 because of ammo costs.