hi guys and gals, ive been looking online to buy an OEM air filter for my SK1 650s, some i see are £15 and others are over £40 so im a bit worried that if i go for the cheaper one that it might not actually be an OEM part and just some cheap copy, and obviously i dont want to but the expensive one and get ripped off

so would be very grateful if someone could either give me a link for a reliable source of OEM parts or tell me where they get theirs please.
and also am i right in thinking that the SK1 filter is a tall doughnut shape and therefore different to the sk4-onwards which is a flat rectangle?
thanks for any help, i watched you tube to change the oil and filter but cant seem to find as much info on the air filter, at least not the SK1 anyway..
thanks again