Forests seem to spawn them mostly, but they still only spawn about once per minute, and usually don't drop them. Would being up on a floating island work better, since there is only one enemy (well... two for Hard Mode) that can spawn? I mean, being on the ground can spawn so many kinds of enemies or creatures like fish or bunnies.
Best to farm them pre hardmode, so you don't have a bajillion other enemy types cluttering up the spawn. You don't want to go to sky islands because then mostly just harpies will spawn. --- void in your head, void between your ears, void everywhere
Try taking a battle potion at night and walk around near yourhouse. That's what worked best for me. If that fails I may have a few laying around I could spare or trade for something --- XBL / PSN : SuddenClassic
just make a new world and dont turn it hardmode. just keep running left and right on night time you'll have like 20 lenses by morning --- I got 3 bad touches before I got my first good touch. I haven't gotten a single bad touch since and I spent 2 hours trying for one tonight.
You can also dig a simple lava pit trap and stand under it with a water candle while you afk. Just pick a flat area that doesn't have caves around it. --- void in your head, void between your ears, void everywhere