Is thare a way to farm turtle shells?

Well you could make a sort-of keymold farm within the jungle, sort of like this, apologies for the ASCII-art:

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                  #       X       #
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i.e. surrounding yourself with a somewhat narrow rectangle and have some minions kill everything. Then go afk for a bit and maybe after a while you'll have some Turtle Shells dropped.

Also put down a Water Candle, of course. And beware of Angry Trappers, they might still be able to reach you.

Turtle Shells can only drop from Giant Tortoise (5.88% drop rate) in the Hardmode Jungle Biome (both surface and underground).

I personally have had many turtle shells (about 4) after getting full turtle armor and I kind of suspect I'm really lucky or the percentage rate goes up. But any way you can carve out a huge box. Make a small box in the middle. Get full chlorophyte armor and a little leaf should appear above your head. That leaf shoots at potential enemy's . Just wait there and be afk. Occasionally look up to check if you have any turtle shells on the floor. Or you could just start tearing down your jungle and exploring every single hole and gap while fighting turtles. <---- I find that more fun but if your patient you could do the other method.

I built a small house base in the Underground Jungle and had a pygmy set. I spawned the pygmies outside the house, and let them kill all the mobs. (I was also trying to get the Bezoar at the time) I put out a water candle, held a water candle and used a battle potion to increase spawn rates. Specifically I was below the Underground level in my world, so this was pretty deep down, 700 ft or so below surface. The turtles still do not spawn as often as the hornets/moss hornets do, so it takes some time.

Except for the Angry Trappers which can reach you, this is almost an AFK turtle farm. The only drawback I found was that sometimes the pygmies traveled all the way back to my toon, and were no longer on the exterior of the house. The main method I used to stop this was to be able to spawn at least 3 pygmies and put myself in a 2x3 size room like a "widow's watch" as a second level on the house area.