Mustard seeds
The small brown or black mustard seeds used in Himalayan cooking are sold in Asian and Indian grocers, while the yellow seeds found in ordinary supermarkets are milder.
Brown and black mustard seeds are sold in Asian and Indian grocers or by mail order, and yellow mustard seeds in any supermarket. In hot oil they pop and turn nutty rather than sharp.
Substitute:
Yellow mustard seeds will do at a pinch, but they taste milder and less pungent, so use a little more.