Hi Be.moody,
The better options and possibilities I’m aware of in the Toronto area (subject to the guidelines here) are listed in post #1 here.
You’re right that many people use them interchangeably (and incorrectly) but a box spring has actual springs inside it that flex and are most commonly used with two sided innerspring mattresses or with thinner mattresses that need some additional “give” underneath them. A foundation doesn’t have any springs and is designed to provide a much firmer support surface with either very limited or no flex.
Phoenix