On both counts. A) Buy a mask that fits. There are a myriad of sizes and shapes, for a variety of face sizes, and therefore, next to impossible to buy one online.
As to trying one on in a LDS and then buying online, that's paramount to stealing. To suggest otherwise is immature and dishonest. Maybe you should find out what Alex6 does for a living, pick his brain, borrow his equipment, use his expertise, and then spend money on something other than Alex6, just to see how he likes it.
And no, I don't own a dive shop, and I have bought online - stuff I've tried on after borrowing from a friend or likewise.