Earthworms would be a great addition to your frogs diet - good luck with your project Xenopus Express sells a good prepared food that is reasonably priced, but the shipping cost will be very high. I checked with a source in France, but due to international regulations, they cannot ship frog food outside the European Union.

A very good prepared food for Xenopus is Tetra's Repto-Min floating food sticks, if you can find it. My personal opinion is to save your money and feed your frogs earthworms or whatever is found locally in your area. Minced beef is OK as a treat. All of my frogs are fed only Repto-Min and they are very healthy and live 12 years or more.

It looks like the closest pet shops that sells reptile supplies are in Durban or Jo-berg, SA. They might be able to ship some frog food to you, but I am not familiar with postal regulations.