You will just have to keep checking the garage sales, craigslist, etc until one appears. It may also be a good idea to see if there is a local fish club in your area. Many clubs have auctions a couple times a year and it is a great time to find items like these.
I recently attended one near my home and was able to pick up two 10G, a 5G and some smaller original Metaframe hoods for a reasonable price. I've never seen them offered outside of a local fish club.