Where? The answer is simple - internet If you want to try henna and get it done before you try it for yourself there is one very talented artist in Canada maplemehndi.com/ (I love her henna ).
As for suppliers there are few, I`ve heard that this one is quite reliable www.hennasooq.ca/ (I`ve never ordered from them, I`m from Europe and I order either from India or Morocco or some European suppliers).
You can try browsing trough instagram for henna artists and really good henna tips
Try searching - many people in many many countries are henna artists. Many of them aare like me - self-taught - internet is a really wonderful thing. Try searching for henna artist or learning hennaing yourself, simple design aren`t that hard