One of the most difficult aspects of raising Platycerium plants from spore is finding the spores in the first place. The only official source I know of is the American Fern Society, from whom, for 50 cents, you can obtain enough spores to potentially supply the world demand for P. coronarium (and various other species, depending upon the vagaries of donations) through 2032. Once you've exhausted the AFS spore exchange, it's time to start contacting individual growers, nurseries, and botanical gardens, many of whom will be happy to supply you with whatever they have available, if you ask nicely. (Please don't resort to the hoary old horticultural tradition of swiping a small chunk of plant that you imagine no one will miss.)

If you find yourself in the position of collecting spores yourself, look under leaves for spore patches that have turned a brownish color, as these will be ripe. Often, a mature spore patch will have a mangy look, as clumps of hairs fall out over sporangia (spore capsules) shedding their contents. In P. coronarium and P. ridleyi, the sporangia, spores, and associated hairs are dropped all at once as a fuzzy brown mass; in the other species you can collect spores by gently scraping them off, or by cutting off a piece of leaf and catching the spores in an envelope as the leaf dries.

You can clean the spores by sifting them through a very fine mesh (most of the old sporangia walls and hairs will stay behind, while the tiny spores will fall through, if you manage to get the right size mesh). I don't bother, as a bit of residual debris won't affect your results.

Platycerium spores seem to remain viable for some time (on the order of years), though a couple of my sowings of spore of unknown age have failed, I suspect because they were too old. Storing them in a refrigerator is probably best. Platycerium wallichii, alone in the genus, has green spores, which typically indicates that the spores are unusually short-lived.