Kitishane covered it pretty well. Mules are often hidden in convention stock so aside from trading for one, the most likely way to grab one is via a convention pull. Typically teacup mule eggs are set out on the table in the same way as regular wyrms so you cannot guarantee a mule pull. Occasionally, mules are set aside from regular stock eggs so you KNOW you are getting one. (Such as the case of a mini sized mule picked up at a convention or a batch of mules like a guest sculptor nest)
Sometimes they also show up here on the form via a sale thread from HopefulMonster or via a special event such as an auction. As for "buying a slot" to get a mule, most often that is not an option. Custom or semi-custom mules are usually involved in special events/auctions too. (Such as the Anthrocon scavenger hunts)
These special events each have their own rules and announcements