There are several lists on the web for good calcium foods. Do a search
to find one of them (I don't remember whether my web site has a link to
one of them. It might.).
For carnivores, just dip the meats into the calcium powder before giving it to
the turtle. That will work just fine.
Good sources for calcium are, of course, green leafy vegetables. If
you feed pellet food or some dog/cat food, your turtle also gets
enough calcium for maintenance.
I usually put a cuttle fish bone in with
the turtle. You can get those at pet stores that carry bird supplies.
Most turtles like to chew on them, and they are pure calcium.
To help your turtle recover, you can also buy a calcium/D3 powder (the vitamin
D3 is important, since it is essential for calcium absorption; Vitamin
D3 is produced by the turtle with help of sunlight, but many turtles don't
get enough sunlight, and so either a UVB producing fluorescent light or supplemental D3 must
be given). Put the powder on any food you give the turtle. Not much is
needed, so if he gets a bit with every feeding, that's plenty. Do this,
until the shell has returned to a normal hardness. Then maintain the
calcium through the new diet you are feeding.