Petsmart does carry them, though I've only seen them carry San Francisco brand mysis. It's not as high quality as PE Mysis (smaller shrimp, less shrimp density per cube) but it will do. I actually have to drive 45 minutes to find PE Mysis around here so I'm using the San Fran stuff right now myself.

If you have a LFS they may carry it though. I wish the LFS around here did. You could order it over the internet from Dr Fosters & Smiths ($$$ pricey) site or ask your LFS to carry it if you have one near you.

BTW, when your frogs are small feeding them mysis shrimp directly isn't bad either. It's a really good food to add variation, in nature african clawed frogs do eat a lot of small arthropods such as these though as they get larger they tend to outgrow them as adults (hence why I mix with reptomin).