There's the server shop, /buy gunpowder #, but that's usually sold out these days due to the amount people buy for fireworks.

You can get gunpowder from creepers, but mobs don't spawn that well right now so that's not a good source. You can also get them from ghasts, but again that's rather hard to get a stable source from.

There is one source that is available on this server however: Gravel. With MCMMO Excavation skill you get access to gunpowder drops from gravel at level 75. This is perhaps the single-best way to get gunpowder. But you can't get it from placed gravel, only gravel you find while mining and such.