Yes, you CAN. In Some states, they are made illegal, such as California, which bans all exotic animals.
In most states, however, you have to get a permit, which requires 2 years experience with the animal before you get it.
In a couple states, they're completely legal, you just need to get an import permit, and have a signed health certificate of the animal.
So first, I suggest you check you local state laws- you could try by contacting your local DNR.
To answer your question: You would get them from breeders of them. There are actually quite a few... if you give me your state, I could find one nearest you.
You can also look up "red fox breeders" or "red fox for sale", and probably quite a few should show up, but there are a few internet scams out there, so be sure to check to make sure they're not.
Also, you may want to make sure the breeder you are getting one from is USDA licensed- they don't have to be, but it assures you they're reputable and that they most likely sell good pets, and that they aren't scams.
Even though it's not part of your question, I'd like to show you some negatives (I want to ensure that you will do your research for this animal before you get it if you decide to get one.):
As stated before, they do stink, very badly, it's a strong musk that smells a bit like a skunk. And as stated, they Could be removed..but it's a very risky process- and isn't as simple as de-scenting a skunk... and although it make get rid of the musk, it will not eliminate the smell of the urine, which is an even worse stench... Which also brings me to tell you that red foxes mark..a lot. So prepare to have your floor, food bowls, bed stinky and sticky with urine...
It is possible to get them potty-trained, but they will most likely still mark. And you can get them nuetered or spayed, which will reduce the smell a little, and reduce marking a little, but you'll still run into problems.
They also tend to be destructive animals if you don't know how to entertain them properly - they are very active animals, and if they have nothing to keep them busy, they'll probably end up just doing what they would normally do, and just start digging or something, but opes, that's your carpet... but if you keep it busy with toys or you playing with it, then it shouldn't cause any damage.
They can end up being pretty pricey... Although the initial cost of the fox is about $300, you will also need to build an outdoor enclosure for it, which can cost between $200 - $800, depending on how much you want to spend, and how good you are at finding deals, or if you're willing to build it yourself, and it will need to be covered top and bottom so it can't get out, which isn't how normal kennels or something come... and the cage and crate and other supplies add up to about $700... and the food can get pretty pricey, so you'll be paying about $50 a month for food... and the vet bills may add up too, but that can vary greatly in price...
And speaking of food, you'll also probably be making it's diet every single day, so you may not want to do that. Although, you could just give a high-quality, high-protein dog food..but it's suggested that you add fruits and vegetables and supplement it with meat...
I think that's probably good for now. If you'd like any more information, or if you'd like some breeders, just drop me a message.
Good luck! I hope this was at least slightly helpful.
I've been wanting a fox for a few years now, so I've been researching them for about 2 years.