Keep killing slimes in the standard mines, the secret woods, and turn on monsters on your farm (at the witches hut) if you haven't already so you can kill a few each night there. Get the slime ring from the adventurer's guild (after killing 1000 slimes) so slimes can't hurt you or inflict slow on you. Then, you can into the higher levels of skull cavern (1-25 aren't bad) and just worry about the flying serpents (face them head-on when fighting them) while mining for omni-geodes. Bring as many bombs as you've managed to get a hold of, a few high health items, a stack of low/mid health items, a staircase or two (when you just need to get out of a floor), and as much coffee as you can make beforehand. Plus, of course, your weapon and pickaxe. ^_^
You can also get omni-geodes in the standard mines from level 40 onward or in the quarry, just usually in lower numbers. You can purchase them from Krobus or Sandy on specific days.
Make sure to check the quarry on good luck days for a mystic node. I just got a prismatic shard and two iridium ores from one.