The thing is, the mortar & pestle are made for that purpose & nothing else: there's not really a better alternative, if what you're after is a fine powder - modern technology has refined the m&p somewhat, but it's just hard (and annoying) to try & fake the real thing.
I remember one guy here used two coasters to grind things smooth, but obviously, YMMV.
I have two: one old-style stone, one modern composite (for dentists & pharmas) - and the one excels where the other fails...and vice-versa. I need them both.