Try Kartuz, Taylor Greenhouses, and keep an eye open for eBay sales. Steve's Leaves may have it as well (looks like they have it in stock). Do keep it in a terrarium in winter though - it needs high humidity.

https://stevesleaves.com/store#!/Begonia-bipinnatifida/p/17557717

Thank you so much for this. Will follow up with Steveès Leaves. (Hoping they will ship to Canada.)

Good luck on your quest! Being in the USA would make things easier but some vendors will ship outside for extra dollars.