If you can't find them at a local NAPA or the corner chinese parts store, Redneck Trailer Supply off the 15 frwy has most everything you can imagine.

For what it's worth, I always put a small slice in the rubber cap so when they get real hot they don't push the cap all the way off and throw grease everywhere.  Yeah it traps a little water but that substantialy evaporates on the way home across the dez.  A slice rather than an actual hole keeps most water out still.

You should replace them with Trailer Buddy's. The size on them is actually 19.80 and they shouldn't come off. Plus they have double o-rings and shouldn't be slinging grease. They also got a trick little cap that looks good. You shouldn't need a bra.