Generally the best way to get custom maps is by callvote. Rotations don't usually get to custom maps. It is very easy to get non-default maps on PureTremulous, and it is relatively easy on D*S (depending on which D*S server it is, share might be on). Getting custom maps on AA (share on) is difficult. I haven't played on Twilight Imperium much but it seems pretty similar to PureTremulous.