Custom items are the special wands, nets, XP pouches and trench tools that other modes hand out as crate and shop rewards. In Prison none of them are currently obtainable: no crate, shop entry or reward gives them out.
Prison has its own special items instead, covered on their own pages:
The pickaxe and its enchantments are on The Pickaxe and Prison Enchants. Placeable blocks are covered on Custom Blocks.