Depending on the recipe, I would substitute regular breadcrumbs. Or if you're making them yourself, just process them a little courser than you normally would. Also, you might want to try crushed saltine crackers, if you've got them in the pantry.

AntoniaJames, I have used my own bread crumbs in place of panko but I toasted them first--just lightly in a low oven until crunchy. You could also use rice krispies--especially if you're trying to make something that's gluten-free.

I do a combination of both mrslarkins and drbabs recommendations. I grind my breadcrumbs coarser than I normal would and toast them lightly in a 250-300F oven or in a dry skillet over low heat. They've worked just fine as a substitute for panko.