need tyre size 205/50/zr16 90 ?

hi im in need of tyre for my astra but i cant aford a new one just looking for a part worn one size 205/50/zr16/90 due to my mot due my spare is wrong size 15 inch so i cant use it so need a tyre for my 16 inch alloy so anyone selling one cheap cant pay no more than twenty but cheaper the better as i havent got alot of money so any can help please email me many thanks