There is little need for a capo on the bass in everyday playing, except for modifying the timbre of the instrument or, as IronLung mentioned, for experimenting with different techniques. You might also find it useful if you learn a bassline, but someone wants to play it in a different key and you an't be bothered transposing. But my opinion is that you should learn it in a new key as a matter of self-education. As for a model, I've seen people recommend this one, but any capo should fit a standard 4 string bass:
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