I bought some of the 2 1/4 in ones from chiefreef on ebay. They are 5 bucks for 17 plugs plus shipping. Heres a link to his website Large Frag Disks (17 Count) - Chief Reef
I use the Advanced Reef Concepts plugs from Scott that Murf was talking about. The thing I like about them is they start to get coralline algae in 2 to 3 weeks. Other brand plugs just don't seem to do that,especially tiles from Home Depot.