I have a special "gross stuff" coffee grinder (cheapest I could get) that I use to powderize nuts, grains, rice, dried bugs... you know, things a roommate won't allow in the regular coffee grinder.
You can chop fruit small and dry it in your oven at 350 with the door cracked open. Heat til they look dry and are barely sticky to the touch.
Frozen bloodworms, krill, mysis shrimp, brine shrimp & daphnia can all be aquired in convenient blister packs in the fish section of a (good) petstore - they'll have 'em in a fridge.
Talk to the reptile peeps at your store & see if they'll let you have some snake sheds. This one guy at my store gives me some, and the crabs are grateful to him, whether they know it or not. :roll:
Dulse, kombu, arame & norri are sea veggies (ie. Kelp, seaweed) - available at healthfood stores, east asian grocery stores (lots of those where I am, but maaaaaybe not super common in most of the states. ...) and anywhere rich people live &buy groceries. Dulse (a red seaweed) is great for colouring, and can often be found in flakes you can scater on the sub to promote foraging.
Timothy hay & alfalfa from the pet store rodent section will be appreciated. Millet sprays & crushed oyster bits can be found in the bird section.
Literally every section of the petstore is more useful to crabbers than the crab section.
Dry oak or maple or (if you live where it's warmer) palm leaves make great leaf litter - a significant part of their diet in the wild.
Birch bark too. In the spring I find birch catkins, maple buds & the flowers of safe-list trees to add (I feel like spring should be special.... even if it's not the same spring that the crabs had back home).
Rose hips, petals and leaves are good later in the season. Mine really lIke honeysuckle blossoms. .. i think its because they smell so good. Burning bush leaves are another popular item & don't forget dandilion - flowers, leaves, roots - all good... just make sure you aren't collecting from fertilized/pesticided/non-legal-to-collect places.
Check out the frozen section of your local grocery store - if you find shrimp, octopus legs, clam, oyster, or crab claw WITHOUT PRESERVATIVES ADDED, grab 'em. (My crebs really like octopus legs. They aren't so hot on calamari rings, so I've stopped getting g those.)
Oh! And at a super froofy "we sell free-range organic dog food" pet store I found, they had dried salmon, whiting, fish tails, and duck wings. The dried duck wing, bone and all, has been the single most fought over & dragged about food in both my crabitats.... even beating out octopus legs!
Check out dog places to see if desiccated fowl is available... but ask about and avoid smoked products.
and congrats on becoming a crab owner! 
welcome to the HCA! 