Is it relevant to post about a frozen food supplier here?
I've been very pleased with A&N. Their service is good, their website very clear (they give a list of approximate weights for mice and rats), the woman who runs it is very helpful over the phone and will tailor the order to a customer's requirements if a boxful of rodents is being purchased, and their prices are so low that if you have freezer space to buy in bulk or can share an order with friends, the rodents are still the cheapest I've found, despite delivery costs. (Starts at £20 for a small box, but you can get plenty in a small box and a rat weaner costs less than my -Yorks - local price for a pinkie.) They also do multis and chicks.
They are based in Suffolk; if you live locally you can pick up yourself, even cheaper.
First heard about them from Eeji on TCS (Ian's Vivariums.)