I'm glad you're going to give them a good home! So many people shove them into tiny bowls and tanks. It wasn't right for someone to dump them on you, but it's great that you're still going to keep them and do what's best for them. ^^
I can't help with a pond, but for cheap, temporary housing you might want to look into plastic storage bins? They're not really built for holding water so you might need to buy two and put one inside the other to make it stronger and otherwise mess around with it a bit (I've seen people use bungee cords to keep them from bowing out). Even the biggest storage bins might end up being a bit small for them, but they'll definitely appreciate any extra space you can give them!
Also, stalk craigslist. Sometimes you can find really cheap (or free!) tanks and supplies on there. You might get lucky and save a lot of money.
I wish I could help more, but I know next to nothing about goldfish and only have small tanks. (And a pond in my backyard, but nothing living in it). Don't forget to keep us updated!