Oil capacity should be cast into the gearbox side cover somewhere around the oil filler (typical practice on Japanese bikes).
The oil filter is behind a bolt-on cover at the front of the engine, behind the headers.
I'll leave the best place to get one to someone with a 250, since we never got them in the US - and since it doesn't use a spin-on. Most likely, you'll be restricted to a place that sells Suzuki parts. Should use the same filter as a 400 Bandit.