I normally either preorder mine online through the Disney website or check to make sure the Disney Store or Safeway have them at the same price and buy them there. I prefer the ones we buy at the store because they are plastic instead of paper though.
Don't know if your Costco has the 4-day packs or not. Ours does and it's a great value. Also, I sell my extra tickets on eBay all the time. I also buy from there quite frequently. My next best suggestion would be Disneyland.com, because they are discounted compared to the gate price.
I bought mine through Ares travel and will pick them up at Will Call, they were about $5 cheaper per hopper than buying them at Disney website. I can't post the link but if you google ARES travel, or Mousesavers has a link to them under Disneyland tickets