What has been suggested by many travellers is to make a sleeping sheet which is one sheet folded over and sewn down the side. It doesn't usually take up too much space. Or you can hire sheets from the hostel for a small charge. You can also hire towels. I wanted to self-cater to save money and youth hostels don t offer those facilities.
Many hostels do have self-service kitchens, cafeterias and dining areas.
The best way to check the facilities of a particular hostel to see if it will suit your needs is to check the Budget Accommodation Guide published by Hostelling International. This lists all their affiliated hostels and their location, facilities and requirements. You should be able to get a copy of the guide at YHA offices and large bookshops. My flight arrives in the middle of the day and youth hostels are closed then.