If you look on the CSJ website it should have a list of stockists for you to find your local one, they might not have ferret food in stock but can order it in. I think they deliver off the website too.
It's 18 -20 quid a sack.
I use a few frozen pinkies now and again but not many of my birds are interested in them, the siskins I had liked mealworms but the others weren't bothered.
Broccoli, petit pois and soak seed are more popular.
A lot of people keep them as pets and it's up to them what they keep, I like a smallish ferret but they are still big enough to work.
If I was offered some true micros and I had cage space I'd have them as pets.