Depends. The one with villager sprites or the GFX sigs like mine?
There are many shops in the TBT marketplace to get a sig with villager sprites.
Here's one I highly recommend:
Pengutango
For the GFX sigs like mine, there are many people in the museum who do them for free, and some in the TBT marketplace who do fantastic ones for a small fee. Here's one with amazing sigs:
Kairi-Kitten and another one:
London
To make them yourself, you can use things like Photoshop, GIMP, or a photo editor, really. All you need is imagination, common sense, basic skills, advice, and patience.
If you want to make GFX sigs, I recommend looking at this
sticky thread.