Considering that if the boot has deteriorated and cracked, it could well be that foreign materials/liquids have gotten inside and might be deteriorating the internal workings too--maybe even allowed some grit to be embedded in the wear surfaces that will cause quicker wear than normal. Considering that if you already have to disassemble things to install the new boot, you're already most of the way "there" to replacing the joint, might it also make good sense to go ahead and replace the joint with a new one and be done with it? A small investment might lead to greater piece of mind in the future too, even if you need to figure in the cost of a fresh front end alignment into the mix too.
Otherwise, you might find some via the "HELP" parts rack at most auto supplies, or listed in the MotorMite catalog. I somewhat doubt that they will be of the same quality rubber that would be used by Moog or Perfect Circle or similar. Maybe you need to find an auto supply with an older catalog and someone that knows how to use it, with all due respect? In the current times of large scale consolidations and buyouts of auto supply stores and distributors, finding an old line, entrenched auto supply that does a large volume can be a real find, especially if they have "motivated" employees AND a stack of old paper catalogs archived somewhere.
One distributor told me that Moog had a warehouse of nothing but New Old Stock parts they used to make. Not everyone knew about it either, but those that did dare to go ahead and order some of the older parts from them got things they figured were long gone. Only restriction is that if it is bought, all sales are final. I'm not sure if it's still around or not, but it might be worth a try.
As "universal" as NAPA might be, the stores might have some regional restrictions of where they can get some of their parts. For example, a store owner in East Texas told me that they would not let him buy his AC-Delco items from a warehouse distributor in Dallas (which had more stock, was cheaper, and gave better service) but instead had to use a distributor in a Texas/Louisiana border town (smaller, less stock, poorer order fill rate, etc.). Similar situations might exist with other product lines they carry too.
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