Hi! Depending on how adventurous (and how do-it-yourself) you are, you can either get a kit that includes a pre-formed mold and pump, or you can simply buy the plastic liner and add a pump separately. If you are going to have fish, then you really will need a pump.
I've found that Home Depot and the similar home fix-it stores have decent prices on pond stuff...and the wholesale clubs sometimes have the entire kits at great prices.
Given how far north you are, you will probably want to dig it at least 3 feet deep at one end if you want the fish to overwinter in the pond...otherwise you will need to bring them indoors.
A 6x6x3 pond would hold about 800 gallons and could home 40-60 fish. You would only need to buy half a dozen though and they would fill out the rest of that space for you.
You will also want to make sure you don't get goldies that are egg shaped, like orandas, moors, etc. The fancy bodied goldies don't do as well outside and definitely need tropical temps year around. A pond this size also wouldn't work for koi.