There are a few different "standards" for bridges, and a lot of repair shops do get blank bridges in (looks like a bridge, but top and feet are very oversized and are shaped from there) instead of starting from scratch.
Carving a bridge even from a blank is a lot of work. Obviously the feet have to be matched to the top of your bass, and every bass is a little bit different. Getting it to fit perfectly to the top is an art, and then you have to match the top to the radius and/or bevel of the fingerboard. From there, most shops and makers do a considerable amount of other shaping of not only the thickness, but the wings and cut outs as well. The mass of the bridge can greatly alter the tone.
I am not saying it can't be done. I imagine it would be a lot easier to go from a piece of wood to a blank vs. a blank to a properly fitted bridge.
Best of luck.