Rare marker guide:-
www.pbnation.com
As for how they come into being, Planet work with the team on the design artwork, as far as I know.
Team guns go into general circulation when the team is given (or has to pay for) next season's gear.
Some teams are "given" markers (depending on who they are and how successful they are). So the players sell their spare guns to pay for rent, air fare, whatever.
Others have to pay X amount for next season's package - say, marker, loader, goggles etc. So they sell the previous season's gear to fund the next season.