You can buy calcium fortified water gel for the crickets to eat/drink. Also, if you mix calcium powder in with the cricket food they will be gut loaded with it.

Word of caution: overdosing your crickets with calcium will likely kill them over time. You should only gut load them with calcium products 24 hours before feeding.

Roaches are even better for this than crickets. They can hold a lot more food in their guts and will survive much longer than crickets. Also they are more nutritious and have less shell and less fat than crickets.