I have shown both my dogs in CKC obedience. Both Emily and Clipse have their novice titles. I love it, it is so much fun. Right now we are working on the retrieve to get ready for the next level. The bad part is that I'm a lazy trainer so who knows if we will ever be ready, LOL Even if we never are ready to compete in Open obedience I am going to take them to the USA to get their AKC novice titles next year.

The ASCA (Australian Shepherd Club of America) also allows mix breeds if they are spayed and neutered I believe.

And then there is AMBOR, an organization just for mixed breeds.
It stands for American Mixed Breed Obedience Registration

Hope you find something near you, and that you continue to have fun with it.