Any kayak store like MEC sells good helmets but they are sometimes pricey. You may be able to use a ski helmet if it is not too big and clunky. Even some minimal bike helmets are sometimes used. Just avoid big helmets that stick out.. remember that in some crashes the helmet can scoop water and pull on your head so you want one that closely conforms to your head.
I always wondered if a climbing helmet would work. Specifically the Black Diamond Vapor. Noticed someone using this helmet last summer at the Squamish Spit. Never had a chance to ask how it was working for him. Lots of drain slots!
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That would work if it's comfortable. When learning to foil you sometimes bonk your head on the board or perhaps the foil. It is usually a relatively low impact force (compared to climbing). Just get something comfortable because they only work when you actually wear them>