Cube them. You need a regular (medium-sized) gem (and 3x healing and 3x mana, any sizes) to make a full rejuve, or you can burn chippies to make smaller rejuves (chipped gem + 3 healing, +3 mana, any sizes), then cube 3x rejuve to full rejuve.
Small rejuves drop quite often, cube all of them you find. That usually leaves me with more full rejuves than I need. I usually even make a decent profit (750 gold for each) by selling full juvies once I have a good supply.
Thanks for the response, jiansonz. I always collect and cube the small rejuv potions that I find. I emptied my stash to defeat Duriel, so I wanted to know how best to find more. I missed the gem recipes, thanks for pointing that out.
I ended up doing NM Tristram clears to get small juves and normal gems. It may have the best boss monster concentration, and is a quick clear (I tried Lower Kurast first, too slow) Tristram gave me 1-2 normal gems and 4-5 small rejuvs per clear. I averaged ~3 full rejuvs after cubing, not bad at all.