You get goblin stamps for helping out your chapter - in your case, SoMN. SoMN has a
donations page that shows how much they give for donations. You can typically also get goblins by helping out in less material ways, but you'd have to ask one of the staff folks about specifics there. You also get goblins when you NPC an event.
Once each month, you can pay 30 goblins to put an "experience blanket" on your character. The goblin blanket is equal to half the amount of experience you get for a typical weekend event. Each blanket will add an amount of XP to your character equal to your current BP total. Adding XP adds to your BP - each build point is worth a certain number of XP based on a scale that increases along with your build.
In addition to blankets, you can use goblin stamps for a few other things:
Goblin production - you can spend up to 50 goblins per day of an event on production items at a rate of 1 goblin stamp per production point. You don't have to have the skills to make these items, so if you need a couple spare quivers or some cure light wounds potions, you can spend goblin stamps and receive those at the event. IG, you either found them or bought them between events.
LCO (Local Chapter Only) magic items: specifics are
here. As you can see, you can also get formal scrolls and things like that.
You can "buy back" deaths. Each death on your character sheet means you'll be less likely to resurrect successfully the next time, so you can give up some goblin points to get rid of a death. You can't buy back your first 2 deaths and it gets more expensive each time you do it.
Money! You can trade in goblins for copper at a 1:1 ratio.
Event blankets - if you miss an event, you can pay 30 goblins per day to receive experience as though you were at the event.
I think that covered everything, but feel free to ask questions if I wasn't very clear or correct me if I'm wrong.
