all site events, updates, etc (including raffles) are always announced in
the bulletin board, with participation instructions posted in the thread there or redirected to another thread elsewhere on the forums for information
indeterminate, but some that have always shown up since they began are the leftover tickets (fair currency) raffles for the end of the fairs and the x days of christmas raffle that
@Oblivia has hosted for the past several years now
the tbt fair is a reoccurring major site event with a wide variety of events, contests, and even some gaming competition that allows for both digital (including collectibles) and physical prizes to be bought and possibly won. for an example of what the fair entails, you can check out the
2020 fair closing ceremony thread in the bulletin board. also, the other closing ceremony threads as far back as the 2013 fair should be there too (so far, the fair has ran in 2005, 2013, 2014, 2016, 2017, and 2020)
at a pretty steady pace, with more major events like easter, halloween, christmas, and the fair typically having more to offer. minor stuff might just have 1-2+ new collectibles, but it's overall a pretty steady release stream where you could probably expect at least one new thing every month or so
it also varies. some are bought from the forum shop (with tbt, or for most event-released ones, with the special currency gained through participating in the event), others can be given as prizes through a raffle, staff favorites award, or as a contest prize. also, some are given purely as a participation prize for an event
most contest prizes as well as staff favorite collectibles are unable to be traded to others, but for the majority of everything else, they can be traded/sold to other people for other collectibles or tbt. or of course, someone could just gift it, like heyden did with your yellow candy or make their own giveaway raffle