http://myintimacy.com/collections/swimwear

No Canada locations, but they have online ordering.
Also, yes, very expensive.
However, they're highly recommended.

I went to a store when I lived in Atlanta. This was back when I was 150 lbs, and was asked...well, not asked but not-so-subtly encouraged to try elsewhere, because I was too small (definitely the only time that ever happend to me!).
I believe the sales lady's exact words were "You see we mainly cater to women who have large breasts."


Also, I agree, no one looks good in a tankini and they're terrible to swim in because they ride up. You might as well wear a regular bikini top anyway because all of the material is going to end up bunched in your boobs.

The thing about bikini tops with underwires not existing...where the heck are you shopping? Abercrombie? Justice? Baby Gap????
Even PacSun, Charlotte Russe, Victoria's Seceret, and all those other stores aimed at skinny and/or young people have bikini tops with underwires.