overstock.com

Also check big box stores. If you have an IKEA near, go take a look at theirs. I was pleasantly surprised at the quality when seeing them in person.

We had all three of ours built by a local guy, via word-of-mouth referral. On the plus side, we were able to "copy/modify" a vanity we'd seen which not only made it way more affordable, but it also gave us the ability to tweak the design (we made all three taller than the original, which not only made then more ergonomic aka; back friendly, but also helped to maximize storage capabilities). On the down side, while the guy building the vanities was talented, and a really sweet guy, he left us "high and dry" more than once during the process (he messed up two drawer fronts, and we waited months without word or call-backs, re: status of the replacements, same with mis-measured drawer boxes, and incorrect drawer glides as well on the second two vanities).

If you can find someone reliable, it's certainly an avenue worth considering.

I bought a 48" vanity from MasterBath. My kitchen cabinet guy added an inch on each side to go wall to wall in our guest room bath. This was cheaper than he could build one. (This was 15 years ago, but there may be similar online.)

RTA. Ready to assemble GHI brand cabinets. Just google to find a supplier in your area.

Way to sneak your place in with Wayfair and Lowe's! LOL!