You have at least 2 possibilities, depending on your exact needs:
1- The preferred method is to Import the models from the app they reside at, like in:
from foo_app.models import bar_model
2- You could also have the models you need to share created at the project level thus making them available to all and any of your apps (this method is rather frowned upon)
3- If what you need is not the share the data among your apps but the structure of the tables, you could also use "abstract base classes". and use use them like in the 2 previous options.
HTH