I've found that the best source for me locally for lab blocks is the feeder breeders at the local reptile show. I get Harlan Teklad 2018 lab blocks for $13 for 33 lbs. In the pet rat world Harlan is considered to be the best quality lab block, with Mazuri coming in 2nd. Mazuri goes for $18 for 50 lbs.
Check your local feed stores for Mazuri 6F 5M30 Breeder rodent block. Odds are they won't have Harlan as Harlan doesn't deal through retailers. Mazuri is made by Purina so it should be much easier to find.