Ferrets are best kept in pairs. A single ferret gets lonely and depressed. If you absolutely can't have at least a pair, consider adopting a behaviorally single ferret from a shelter. I have a behaviorally single ferret. The only reason she's a single is because it would be mean to make another ferret live with her. Since she was taken from her mom so early (most pet store ferrets are weaned and separated from their mom as early as possible so they can be spayed/neutered and shipped to the store while they're still babies), she never learned normal ferret social behavior. She is so dominant, she will beat the stuffing out of any ferret you try to house her with and will not leave them alone. She doesn't know that she's playing too rough, her mom was never there to teach her to let go when the other ferret squeaks. I think she also may have been bought and kept as a single ferret as a baby, which would have made it worse because she didn't have any ferret companions at all until she got to the shelter as an adult. Perhaps if she had grown up in a group, she would know how to get along. I feel really bad that she has to be a single ferret, but there's nothing I can do.
I keep my ferret in a martins cage R695 with the ferret "barn top roof" zip tied to the top. She has a ridiculous amount of room for a single ferret, but this would be a great setup for 2 or 3 since it has 3 full levels plus 2 partial levels/balconies. I got it secondhand off of craigslist (a great place for finding good sized cages for a reasonable price. I highly suggest looking for a cage with at least one full level above the base of the cage (by full level, I mean there is a full floor above the bottom, not a little shelf or multiple little shelves). Ferrets get big, and little shelves don't really supply much in the way of room to move around for a ferret. They need walking space, as well as places to flop down and stretch out, and those shelves are too small for that. In addition to good floorspace, the cage should be roomy enough to hang hammocks and cubes. Ferrets are all about lounging in hammocks, so make sure there is enough room to hang a variety of hammocks. For two ferrets, you want to hang at least 2 hammocks, preferably more since they may not want to sleep together all the time. A skinny cage may take up less room on the floor, but if there isn't enough space for properly stringing a hammock, then it's not going to be very comfortable for your ferrets. You'll also need a lot of floor space for food bowls (one for each ferret so they don't fight over the bowl or if one gets pooped in or soggy, they don't have to try to eat out of it afterward.), litter pans, sleep sacks, and toys, while making sure the ferrets still have room to walk.
I find that most of the Superpets cages and the like are very lacking in actual walking space in their cage. They have a ton of little shelves, but little actual walking space. I hate cages with no full levels and a ton of little shelves because it is actually dangerous. Ferrets are klutzes and a fall from the top shelf of a decent sized cage would mean serious injury for your ferret. The expense of a cage is nothing compared to the expense of ferret medical bills. An injury from a fall could easily cost you $500 or more. It's way cheaper to just buy a decent cage in the first place. I would specifically look at a cage and see how far the ferret could fall if it slipped off a platform. I wouldn't buy a cage with a possible fall over a foot.
These would be good:
Ferret Nation Cage
http://www.drsfostersmith.com/Product/Prod_Display.cfm?pcatid=15340&cm_mmc=FerretStore-_-DFS-_-April_2007-_-Cages&ref=4419&subref=AA&N=2007+113918Martins Cages- Rat R695 or RUUD. Ferret F435, F430HR, or any cage numbered 450 or above
http://www.martinscages.com/products/cages/Superpets Cages
http://www.petsmart.com/global/product_detail.jsp?PRODUCT%3C%3Eprd_id=845524441809529&FOLDER%3C%3Efolder_id=2534374302047888&ASSORTMENT%3C%3East_id=2534374302023695&bmUID=1187722050975&itemNo=13&In=Small+Pet&N=2047888+4294966688&Ne=2http://www.petsmart.com/global/product_detail.jsp?PRODUCT%3C%3Eprd_id=845524441777055&FOLDER%3C%3Efolder_id=2534374302036906&ASSORTMENT%3C%3East_id=2534374302023695&bmUID=1187722050777&itemNo=1&In=Small+Pet&N=2047888+4294966688&Ne=2
