OK, my oil filter is a metal mesh one, so I just clean it everytime I do the oil... Just bathe it in some petrol and give it a good scrub...
Not sure on your bike though, if its a paper one and needs replacing rather than cleaning.
As for the oil, I get the stuff from super cheap auto... I think its $30 or $40 bucks for 5L but you can buy it in 1L.
My brother in law just uses normal car oil and recons it is fine... I heard that with the wet clutch its not recommended though, so I just stick to the proper 4 Stroke bike oil...