I assume you mean a cable guide, not a cable stop. A framebuilder can easily and quickly remove the remnants of the old cable guide and braze a new one on, but as Bianchigirll says it ruin the paint in the area of the braze-on.

Nova Cycle Supply sells replacement guides for less than $2.00.

If it really is a cable stop, they have those, too. Cable Stops.

Where are you located?