Do you know exactly what you want? You need to know the thickness (1.3, 1.5 or1.8mm) and for a bracelet clasp you would normally fit shoulderless with rounded ends. Only problem with some rounded end bars is the lack if depth at the end, I've ended up using standard shoulderless and trimming the ends slightly with a file to avoid sharp protrusions.
There are other types of pins with rivetted ends which are often used, but again you beed to know the dimensions.
A picture of the clasp and original bar would be helpful, Cousins supply a huge range and I have a good stock myself, but more info is needed! A digital caliper is ideal for measuring, otherwise its nigh-on impossible to gauge the thickness.