canvases and canvasses both possible
http://www.merriam-webster.com/dictionary/can...

If no plural is listed in a dictionary you can assume it's regular. Words ending in S regularly add -es.

Canvasses is the third person singular of the verb Dictionary: to canvass

tipster, im Merriam-Webster wird als alternative Schreibweise zum Substantiv "canvas" auch "canvass" angegeben, insofern is "canvasses" auch korrekt.

Try an internet search with a word for context such as artist, painter, museum, conservation. It would be very surprising if canvas(es) did not win by a wide margin.