There are several "high quality" ferret foods available on the market. I use a mix of Totally Ferret which is manufactured by Performance Foods and Superior Choice which is the Ferret Store's brand. Since ferrets are carnivorous, you'd need to choose a food with at least 34% protein and 20% fat. Also, check the label and make sure the first 4-5 ingredients are protein based(meat, eggs, etc), not grain/filler based(corn, rice, etc)
Here's a list of some of the better ferret foods:
*Totally Ferret(the top ferret food, but really expensive)
*The Ferret Store's Superior Choice
*8in1`Ultimate(don't mistake this for 8in1 Ultra, which is lower quality)
If they are being fed a high quality diet, they will not need vitamin supplements, even though they are sold in the pet stores. Ferretone is technically a vitamin supplement, but it's really only recommended for use as a treat or as a help in giving meds or clipping nails regardless of what it states on the label.
Some people will feed their ferret a high quality kitten diet if they don't have ferret diet available to them. Iams and Eukanuba are two that I can think of off the top of my head that are *ok* to use. However, I only recommend using them if you absolutely cannot find a really good ferret diet(which is rare). Other people also choose to feed their ferrets a more "natural" diet like the BARF diet--Tort would know more about that than I.
Ferrets need to be given free feed--food available at all times.
You can give them thoroughly cooked plain chicken, cooked eggs, ferretone(or linatone), ferretvite, nutrical, etc, as treats. Treats should always be given in moderation as ferrets don't always know when to say no. Avoid fruits and vegetables--ferrets don't have a caecum(sp?) which is a part of the digestive system necessary for the proper breakdown and digestion of fiber. So if you really must offer fruit, please only give very small amounts and no dried fruit, either. Raw vegetables are not a good idea either--they, like fruit, offer them no nutritional value whatsoever and can cause intestinal blockages.
Also avoid foods high in salt and sugar. Dairy products should also be avoided as many ferrets are lactose intolerant.
I will add more info here as I think of it.