Go to the top left of the Three Horns Valley map (north-west) and you will come to a hideout where the Bad Maw boss is. If you can't find it, look for the place next to the Bloodshot Stronghold entrance.

Kill Bad Maw, pick up the Eridium from him and then go into the Stronghold. Choose the Save and Quit option that you are presented with.

Then, go back and continue the same game, return to the Valley and kill Bad Maw again to get more Eridium!

Keep doing this as much as you like!