Cattle cubes are not a good choice and in fact should be avoided. Go with any of the commercial brands designed for deer. Purina is usually available in most places. Another economical choice that has consistent quality and has grown as many trophy bucks as anything is 16% calf mana. Its all they used in the Ms. State research pens.
There are a lot of things to consider when implementing a supplemental protein program and it is very easy just to throw a lot of money away with no benefit. Non target predation, loss to rain, lack of consistency, insufficient saturation just to name a few.