I'd check here at ebay first, don't know what what reel you have
Shimano Symetre spools, Sporting Goods. Great deals on eBay!
The other place you can check is with Shimano reel repair shops, I know most of the shops that I've been too collect them to help out their customers!!!! I like to have two or three for each of mine.
Most new Shimano reels DON"T come with a spare spool, per a customer service rep I talked to at Shimano, and most folks selling them are trying to make BIG BUCKS for them.
Fatman