Where can I buy this Marcus Brothers fabric?

Go to 'quiltshops.com', click on 'item search' and put in rainbow garden. Click on store search and a load of websites will show up that carry that line of fabric. Check a few of them (some just list any fabric with the words 'rainbow' or 'garden', but quite a few came up with the ones you wanted. Fabric Shack was one of them that had it. Look around to get the best prices, tho. Some can have a big difference in yardage price.