Too many to say, really. The "halfsider" you're talking about was a genetic "oops" and not a legitimate color mutation. There is some info on the internet explaining that.
Many of the truly rare color mutations you won't see in pet shops. But then genetically, there are literally so many color combinations that you sometimes come across unusual or unique looking ones. The ones in pet stores are usually bird mill bred at random.
There is a color called anthracite which is a super dark charcoal which is supposed to be super rare, Texas clearbody is super rare, Fallow, lacewing is not real common. Most likely found for exhibition in the English type. I'm still learning more about budgies and their interesting genetics myself.