Nintendo is fiercely protective of its Intellectual Property and is prone to sending cease and desist letters to artists who may monetize Pokemon-related products. So, while there are a few fan-produced and supported projects, the stuff that could qualify as some of the "best" is few and far between. It is also possible, however, that if this article wasn't written in summer, plenty of fan-made Advent Calendars would be available for purchase. The best way is just to keep your eyes peeled when the seasons roll around. Some of the best purchases you can make are from small businesses rather than mega-corporations.