Hi Aluminium trioxide (Al2O3) also known as alumina is really easy to source from dentistry materials shop, dental technicians use different granulations of it for a procedure normally called sanding where this stuff is sprayed with pressure on hard (metal, ceramic or polymer) materials, mainly prosthetic teeth or parts in order to polish the surface. It's pretty cheap chemical but expect to have it in big 3-5L packages and also dentistry stuff are more expensive than just regular chemicals. Just pay a visit to a dental technician's lab and just ask for a cupfull for free, maybe you will encounter a kind enough person to just give it off, ofer them a little money if you think it's appropriate. Hint: you can also use alumina for column chromatohraphy this is it's main use in the organic chemistry lab and also to adsorb expensive catalysts for them to be filtered and then reused.