Where can I get a compatible parasol that will fit the pram and stroller?
Well, yes you will. A universally fitting parasol will fit both modes if it's such that is attached to the stroller frame. There's no Nuna Mixx Next original parasol as far as I know, however.
I wouldn't, however, advise the use of a parasol - I had one and the wobbling and setting up the right angle are a bit annoying, to be honest. A sunshade (e.g., a Dooky sun shade or a Diono Shade maker or anything well covering and universally fitting) was much better for me - but this is, of course, very subjective and your experience might be different.