Honestly, I had a hard time finding them myself. I happened to see some in a field next to a concrete company (don't know why they had them) and stopped and asked if I could have a few. He said sure, take all I want.
I do know that cable companies and electric companies have them. Check your yellow pages. Also, try Craigslist - put a 'wanted' ad on there and also watch to see for any listed. Use keywords like "spools", "cable-reels" to locate them.
Good luck - my goats love to get up on them. We used an old wood car ramp as the ramp to let then get up on them (although my larger goat just jumps up to them), my hubby attached pieces of wood every few inches on the ramp to give them traction.
I got cable spools free from my electric company. It is hard to find someone by phone that can help you, but if you go into your electric company, they will probably no right off who to talk too. In my area they have to pay to dispose of them at the dump, so they are more than happy to give away any that they can. I will try to get a picture of my goats setup tomorrow. But it is pretty much all cable spools and igloo dog houses. I have the dog houses mounted onto of cable spools and the goats absolutely love to sleep in there!