In the days of yore Mike Manghelli would have a nitrous filling station at his pit/the White Goose Bar - to fill his team's bottles. I asked with a smile and a please and he filled a bottle or two for me. Don't count on it, though -- get in touch with him.
Other than that -- I think Stainless is right. A couple of years ago a nitrous vendor was on the salt for SpeedWeek and found business at his pit was quite underwhelming. He hasn't been back, nor have I heard of anyone else having mother bottles (or any size) to fill bottles of hoi polloi.