Yes, there are a few phone items at WDW at various gift shops and at DTD.
But we have found our Mickey Cel Phone faceplates and slip-in holders at various phone boutiques at malls.
In fact the CM's usually comment on our Mickey Phone belt-clip holders as they havn't seen any like them at WDW.
MY DW has a red Mickey faceplate for her Nokia phone bought at a local cel-phone kiosk for about $5.
You can always find Mickey accessories on eBay, too.
For example (don't know anything about the person listing this item, BTW, but having 635 feedbacks says a lot.)
http://cgi.ebay.ca/ws/eBayISAPI.dll?ViewItem&category=35180&item=5711527181&rd=1&ssPageName=WD2V