Use Yahoo! yellow pages to do a search for body shop supplies. Be sure to put your area code in the locations field. I found 3-4 body shop supply stores that sell Meguiars and 3M detailing supplies all within 30 minutes of my house. Any one of them will have a velcro backing plate though it may not be the Meguiars brand. Most likely will be 3M. Also check Lowes and Home Depot as they may very well have the Porter Cable velcro backing plate. The Home Depot near me has them.
Natty