Five inch sanding disks with 1/2" hole

Bought a Makita 5 inch sander (model GV5010) to grind the welds as I weld in replace metn Very light weight at 2.6 lbs. Can't seem to locate sanding disks. Looking for 24 or 36 grit disks. The sample disk has a 5" outer diameter and 1/2" inner diameter.
1) Can anyone tell me a store in the Denver area that carries this type of disk.
2) I see Aluminum Oxide, Ceramic, and Zirconia fiber disks are common. Can anyone tell me the best type of disc for grinding mild steel welds on my sheetmetal welds?
Thanks

That thing seems to have a very limited amount of options on its disks.7/8s is alot more popular.
http://www.grainger.com/Grainger/MAKITA-Arbor-Mount-Sanding-Disc-5RDX2?Pid=search

Do you have a Harbor Freight by you.

They sell different ones. I like the stacked flapper disc. They come in 60 80 and 120 grit and are great for welds.

The ones at the top of this pic.

Lungs

Check out Lehigh Valley Abrasives on the web, they have everything you could possibly need.