The reason I ask is because the hat should match the suit. If your fiance is going for something business looking or casual, then he'll need something like a fedora, trilby, or panama hat. You should be able to get them at department stores like The Bay but it'll be location dependent. You can also try menswear stores.
If he's going for something more formal, like a tuxedo, then he'll need a top hat, opera hat or Homburg hat. A specialty formal wear shop may carry it but considering North America's divergence from classic formal wear, he may need to go the online route to find something suitable. If he's willing to buy second hand, vintage or thrift stores are also an option.