Morrisons is great for stocking up. Ive stopped there many times. You could spend the night in Northampton Marina if you prefer more facilities/ security. I prefer to do shopping at Wellingborough cos its got Tesco-water and elsan there too.
Definetly join FOtrn -they got some great moorings. Can highly reccomend stopping at White Mills Marina too but again you will have to pay to stay!
If you are here for a couple of months you will easily explore the Nene, Middle Level, Ouse and its triburaties.
Have you Imray for Ouse and Nene?
There is an online map for middle level and other info here https://middlelevel.gov.uk/navigation/
Northampton marina site has loads of info -I would sign up for SSA while you are here too- https://www.northamptonmarina.co.uk
Enjoy!!