Heidi,

The nearest calm flat water spot North of perth is Hilary's boat harbour, or just north of the breakwater near Pinaroo point.
Otherwise make the drive down to the river where you will find plenty of flat/calm spots sheltered from the wind.
try anywhere along the swan river near Blackwall Reach (Bicton, Point Walter, Mosman park, Peppy Grove, East Freo)

Starboard SUP do some dedicated Yoga boards, both epoxy(hard-board) and inflatable. A SUP approx. 10" long by about 34" wide board should suit.

Pete

Thanks guys will check it out. :o)

you could also try Mindarie Marina in the northern corner of the enclosed waterway. Has carpark and easy access to the calm lagoon-like water.