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:

                  #################
                  #               #
                  #               #
                  #       X       #
                  #               #
 ###################################################

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).