If you want to "hang" the mirror by the frame, then a weak frame could be a liability. I've built many custom mirrors over the years, probably 3 or 4 for each house I've built. You can have them built from scratch for a few hundred dollars. You can sometimes take a store-bought frame that might be a little flimsy and back it with a piece of 1/4" luan and staple the luan to the frame. That'll stiffen the frame and joints. This does depend on the width of the frame though, it might not work with narrow frames. You can paint the edges of the luan black to minimize them showing from the side. The easiest is what has been mentioned, hang the mirror glass on the wall, then build the frame over and around it. The frame holds no weight an the mirror will never tilt. When hanging heavy or large custom mirrors that I've built, I usually hang them with a french cleat. Should the need arise, the french cleat makes it easy to remove the mirror from the wall. And when on the wall they'll always be secure and level. Mongo

Depends on what look you're going for - contemporary? Then spend the $ now on frameless doors and get them installed. More traditional? Then do a shower curtain(s) and frame your mirrors. It's not that hard to do (I did it with a huge jigsaw puzzle in ds's nursery) after the mirrors are on the wall, as long as there's room. Only thing is you want to paint the back of the trim black so you don't see reflection of the wood where it overlaps the mirror. A mirror will be thicker than the jigsaw puzzle, so you'll need to route a channel in the back of the trim pieces where it overlaps the mirror, or else the outer edge of the trim won't lie flat against the wall. If you don't have the carpentry skills to DIY, ask your finish carpenter to do it - I'm sure it'll be a lot less than $200 per mirror (42" x ? each). Do you have pics of bathrooms so far (vanities, sinks, faucets, lighting installed?) so we can see what style is? Of course, oceanna's solution is much easier to DIY and looks so pretty (but more casual/contemporary). As always, oceanna, your painting is beautiful! I'd love to see your whole bathroom!

I suspect you'll have to go custom because the odds of finding a mirror that size with the right proportions just sitting around waiting to be bought are slim. Also, it's going to be hard to find sources online that have that huge a mirror as a standard item and are willing to ship it...unless they charge a huge amount for crating it.

With a picture framer you tell them the exact size you want and then you pick the exact frame you want. They ask you if you want your mirror with a beveled edge or not. You don't need to settle on what they have already made. You don't need to buy a mirror and take it to them. They make what you want and for no more $$$ than Wayfair.