UPDATE... Walmart Canada WILL NOT accept giftcards purchased in the USA. Home Depot Canada WILL accept gift cards purchased in the USA (however Home Depot USA will not accept Canadian ones, weird).
Again... WALMART NO. HOME DEPOT YES.
As far as Home Depot goes I'm not sure if the value is transferred according to currency rates or simply "at par" so it might be better to contribute money rather than a gift card as the US Dollar currently is higher than the Canadian so the money would go further that way.
I'm sure if you argued hard enough they'd take the US Walmart giftcards but it's probably not worth the fight so I would avoid it.
I didn't check Lowes because we don't have one yet in Ottawa. (Where I live.)
If any other stores come to mind just ask. If they're local I don't mind calling!
