Ford recommends replacing the lower arm, I presume because of the loading it sees. This is true of the earlier Mustangs as well.
Having said that: yes, you can buy the balljoints for all 4 arms at pretty much any parts store.
Couple of ways to remove the rivits. If you have an air hammer, cut the heads off, and drive them out. If no access to air, center punch the rivits and drill them out till they're thin, but be careful not to enlarge the holes in the arms. Then you can cut the heads off with a hammer and chisel, and drive them out with a punch.
For the control arm shaft bushings, I just work them out with the air hammer by driving on the backside of the flange, and reinstall with either the bushing press when I can borrow it

(pays to work with old-timers), or with a vise and a large socket and hammer that fits OVER the rubber, but ON the shoulder flange of the bushing.
Needless to say, safety goggles are a must. I earlier this year had a piece of steel that was being ground come under my safety glasses and landed in the lens of my eye. Not a fun time. Actually, I think watching them get it out freaked my wife out more than me, although I wasn't too crazy about the grinding of the lens to remove the rust ring. I now use only goggles, in fact threw the glasses out.
Anyway, I would ponder this: a local rod shop sells NEW arms with joints and bushings for something around $75 apiece. Hardly makes it worth the time to replace all that stuff on the original arm stampings of questionable miles/wear cycles. That's the way I'll be going next time I replace the suspension on a II.