Most agri merchants will have a selection.
Here's Mole Valley's.
I think if you say "Shepherd's crook" you get the fancy sort that they use in the ring; if you ask for 'leg crook' or 'neck crook' you'll get the working type.
I use a short leg crook. It is the right size to catch lambs round the neck, or a ewe by the hock.
I find the long ones allow a ewe to be too far away from me, and she can create enough of a whiplash I (a) let her go and (b) nearly dislocate my elbow or shoulder!