I think your translation is fine. It is in a casual, colloquial style. (But, 時には　may be better than 時々. 　See below.) Let me offer an alternative, one with a flavor of literary style. 正義を求めるならば、時には自ら掴むべし。 時々= from time to time, with an implication of “routinely.” 時には = at times, with an implication of “when necessary.” 見つける implication: go find justice (somewhere) 掴む　implication: go after it; seize 掴むべし　= You should seize it for yourself. 自分で　colloquial style 自ら（みずから）literary style Others may suggest yet other translations

The latter implies you do it just when you have to.

Other than that, "時々" sounds a bit weird. I would say "時には".

"手に入れる" means "get it" and "実現する" means "realize it".

"見つける" in Japanese just means "find", which must be different from what you wanted to express by "get it". I would say " (正義を) 手に入れる" or " (正義を) 実現する".