I always use crumbles (usually starter,but I think any kind would be fine) and mix in some oatmeal from my kitchen.  I have been doing it for the three years since I started.  I like not having to buy something else to store, just for the mealworms.  They are SO easy!!  You don't have to be picky or even consistent with them!!  I once left mine in a closet in the garage for MONTHS without any care at all!!!  I just knew they were goners.....but there were actually plenty of live ones in there, all stages, so I refreshed their containers with some oatmeal and potato, and they were thriving again!!!  I have always had tons of animals of all sorts, and this is probably the easiest thing I have done for any of them!!