If you can't find mono stashes
anywhere else, you're gonna just have to buy sets. On the plus side, I
would bet that you could sell of any excess mono stashes you were left
with on Ebay without taking a loss.
Speaking of Ebay...I just did a quick browse in ebay and found two folks who were selling Treehouse sets for around $8 - one listing the shipping fees at $3 (with combined shipping possible for multiple sets). While I'm all for shopping at the Looney store, this is a reasonable alternative.