In my experience, parts stores basically always carry greasable versions of our front u joints, or really all u joints. If they don't ask you when you are there buying them, tell them that is what you want.
Beware clearances, however. The front axle outer (wheel end) u joints will not spin near the knuckle with a traditional grease fitting- the grease fitting would hit the axle beam and break off, as I recall So, when you get a greasable u joint, make sure it comes with a low profile grease fitting intended for a grease gun injector needle. Sometimes, I find that the u joints I buy have a traditional grease fitting on the body (cross shaped center piece) of the u joint, but I don't think there is clearance for this in the front end due to angles of axle shaft pieces while steering, so every front u joint I have bought for the front axle has had a low profile needle grease fitting on one of the caps. These clear fine in my experience during rotation, but again you need the injector needle for your grease gun, and you need to install even the low profile grease fitting AFTER you slide the axle shaft through the hole in the knuckle. There isn't clearance in that hole to install the fitting in the u joint and then slide the axle through the knuckle.
Auto parts stores sell a grease gun injector needle adapter that fits right in your grease gun the same way a fitting does, so that you don't have to change your whole grease gun hose.
One other thing to note: I always lock my hubs and drive my truck in 4wd half a dozen times gently during the non winter months in a field or dirt road to make sure that the u joints rotate and don't freeze up. Other than actual use wearing u joints out, they can also fail from sitting stationary, having water get in somehow, rust develops, and then u joint roller bearings and caps freeze up. Of course keeping them properly greased should prevent this but IMO its not a bad extra precaution. On every TTB Ranger I've had or worked on, the front outer u joints were able to move in the steering direction but were 100% rust welded frozen in the up-down direction, before I replaced them