how can I fix my challenge xtreme sander? ?

A cheap Chinese tool I'm told and doubt you'll get spares at Argos price Challenge Xtreme Orbital Sander 220 Watt £23.39 At that price I wouldn't waste time looking for spares.
Either check with Argos if they still retail them or go buy a Black and Decker a tool you know has a spares backup. It pays to hand sand and save for a decent tool like Makita, Bosch, B&D that have spares back up including the correct sanding sheet.
All said when a sheet has no holes cut them in in the relevant places using piece of scrap steel tube sharpened on a grinder to cut the holes. Or buy a set on amazon they are called hollow punches various size set around £7. approx. $14U.S

If you do buy another one keep the old one for cannibalising for spares.

bobuk
April 2014