I'll link a site I saw recently with a lot of online options. I believe learning what the insides are made of will be tough with an online store. Check around your area for custom cabinet makers and get quotes--you may be surprised at the prices and you'll get them to your specifications. I actually found that after I picked my countertop it was great to pick the exact shade of stain for the wood to pick up the right shade of brown in the counter top so it was good for me to order from a kitchen cabinet company. Try a kitchen store to get a semi-custom quote. I just read this on the Kitchen Forum about a cabinet line Aristocratic cabinets:

"We have two shops that carry Aristocratic and we loved their displays. The cabinets are rock solid. We did full plywood with all the bells and whistles and they were still cheaper than any others we looked at. (And even with the walnut, they came in cheaper than the quote from Home Depot for Kraftmaid!)

Here is a link that might be useful: bathroom vanities online source

Maybe you could go to a used furniture store and find vanities that you can rework. Or on Craigslist.
I used the front of my old vanity and cut off a section so I could wall-mount it with 10" clearance from the floor. That is much better for not encouraging mold. I got oak plywood for the sides, stained and varnished to match the front. It looks really nice.
Laura

Maybe you could go to a used furniture store and find vanities that you can rework. Or on Craigslist.
I used the front of my old vanity and cut off a section so I could wall-mount it with 10" clearance from the floor. That is much better for not encouraging mold. I got oak plywood for the sides, stained and varnished to match the front. It looks really nice.
Laura

Just avoid Bertch. They made my remodel miserable with 3 damaged shipments.

I actually had great luck with Bertch for our powder room cabinetry. My mother also had great luck with them for her kitchen cabinets. Sorry to hear that you had a bad experience. I'm hoping to use them for our master bath. I hope our good luck continues!

To elljays:
I don't know how long ago your experiences with them were but my contractors said they have gone way downhill. Literally three shipments left with damages. I ask you is that quality control? I wouldn't push your luck. The driver told us it took one customer 4 times to get it right. I totally changed the concept for one bathroom because of them not to mention the delays they cost me. Good luck whatever you do.

Hi Marc, check bathroomplace.net they carry solid wood vanities in different sizes. Bought 2 from them and we are very happy.

Here is a link that might be useful: Bathroom Vanities