There are aircraft scrapyards where these are easy to get. Search "aircraft surplus", "aviation scrapyards", etc.

it is known as a very dangerous substance and easy to rust

Yes, it is. Aircraft stuctures are regularly 'NDT'd. -That is tested by x-ray or similar devices for structural cracks or strains.

commercial (easy jet, flybe etc.), army (in afganistan) and flight lessons (light, versitile aircraft.)

When purchasing an aircraft you should confirm that all the relevant safety tests have been completed and that all the documents are up to date. It is also important to know how old the aircraft is, how easy it is to buy replacement parts and if the aircraft has had any major repairs.