beef tripe
The cleaned first stomach of the cow, cut into strips or pieces, soft and slightly gelatinous after long cooking.
Beef tripe can be ordered from a butcher and is often found in Turkish and Asian supermarkets, usually already cleaned and par-cooked; for cooking, use only cleaned, pale tripe, not the uncleaned green tripe sold as pet food. It needs several hours of gentle simmering to become tender and gives the broth body.
Substitute:
There is no true substitute; calf's feet or beef shank give the broth a similar gelatinous body, but not the characteristic texture and distinct flavour of tripe.