In the refrigeration trade I get "B" size tanks of nitrogen gas. According to the MSDS it's 99% pure and I know it has no water vapor in it. You buy the tank and trade it in when it's empty. Lasts forever but pricey to start. Refills are cheap.

I use 250ml amber glass bottles and divide my developer between them. When developing I might use anything from 50ml to half of it depending on the developer. I flood the left over with nitrogen. The bottles are available from Photographers Formulary. B&H carries the Formulary bottles but I'm not sure which ones without looking.