NHP Micro-Fill is temporarily discontinued and you may be waiting a long time for it. Hobbylite by Hobbico is a similar product though discerning people like David Duckett find Micro-Fill to be superior. Also, Jet Model Mate by Goldberg appears to be an ok substitute until Micro-Fill production resumes.
Static display planes can use just about anything sandable for filler, but a flying plane needs to use a lightweight product like one of the three I mentioned above.
Poppy, the master filler, uses two fillers and a primer to get a very smooth finish for his static display models as shown
here.