I've done very well in flea markets and physical stores with Conversation Concepts' line of dog products. It's mostly figurines, magnets and that kind of thing rather than the items you're mentioning, but depending on where you're selling you could always go to a printer with pictures and ideas for shirts, mugs etc. and come out a lot better.
CC's retail page is here:
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You'll need to contact them to find the nearest wholesale rep to you.