I live in Belgium. The best ferret food you can buy in Belgian pet stores is Totally Ferret. Which is what I feed my 2 little rascals. It's certainly not cheap, but it is cheaper than going to the vet because your fuzzbutts are suffering from blockages and other issues.
Occasionally I mix it up with some high quality kitten kibble or other fuzzy specific food. Just in case the stores should run out of TF. It's a good idea to mix their kibble at a young age. Some fuzzies can be very picky and will refuse to eat anything they're not used to. Lucky for me, mine aren't picky at all...
Other countries, like the US, UK and Australia offer a whole different variety of commercial kibble. I found a really good comparison of the best ferret food here. Its US based, but the listed ingredients will help you decide what type of kibble qualifies in your country.
If you can't find any of the brands reviewed, go for the premium cat kibble.
Here are a few tips when reading the packaging:
Don't feed your ferrets cans (unless the vet tells you so). The quality of most cans is below par. And the 'sauce' will stick to their teeth, causing tooth decay. I haven't tried brushing their teeth yet, but if I figure it can be quite challenging...
Personally, even though it's not sold in any of the pet stores close to where I live, I recommend Totally Ferret. I've been feeding this brand ever since I got my ferrets some 5 years ago. They haven't had any health problems in all those years (knocking on wood), so I'm sticking with what seems to work for them. It also came highly recommended by my vet.