If you google "RTA cabinets" you will find a number of sites selling vanity cabinets that you can put together in numerous configurations to end up being 90". As far as the top, you pick the material you want, i.e. granite, marble, etc and have the fabricator cut it, edge it and cut out the sink and faucet holes. We have a 93" vanity two 34" sink cabinets with a 25" vanity in the middle. We were going to replace it with RTA cabinets, but I refinished it and am putting a shelf in the vanity space, will use the top half for towels and put a hamper under the shelf.

Most cabinet companies have a vanity spec book too since vanity cabinets come in 18,21, 24 inch depths and 32 or 34.5 inch heights. They are priced similar to kitchen cabinets so it depends on the cabinet company and door style/finish you choose. The price also depends on whether you want basic sink base cabinets or more. Drawers, legs, inside fittings or special finishes jack up the price quite a bit. I would estimate a basic 36 inch vanity in a moderate priced door style without drawers to be around $700 in Schuler. I have the Schuler (available at Lowes or at independent Medallion stores) spec book and they have either single piece vanity combos in those widths or you can build your own cabinet combinations and screw them together ourself. You could then pick the individual cabinets yourself and even mix up the depths if you want one of the cabinets to protrude a little. Make an appointment at a cabinet store that has a cabinet line which includes vanity cabinets. It would be a quick appointment since you are only picking 1 or 2 cabinets per bathroom.

Normal kitchen height counters have become much more common in bathrooms in the last couple of decades vs vanity height. When I replaced my vanities, I built them to finish at 36 height in both upstairs bathrooms. Neither are 24" deep though because of space constrictions. If the added depth is a problem, you can bump out the wall or mirror above. (But be sure to leave faucet clearance space.) Ditto about using the widest drawer stacks you can fit. 30" vs 2 15" stacks. You gain the space require by one set of drawer slides (if side mounted slides are used vs bottom mount) PLUS two cabinet sides. And, the extra wasted space if you are using face framed cabinets vs frameless. You will end up netting quite a bit of extra drawer space. Extra bonus because one 30" cabinet should price out for less than 2 15" cabinets will. I don't like towers anywhere near water. Have seen too many pix of water damaged tower cabinets when installed close-close-ish to a lavatory. That, and I hate the trapped feeling. As mentioned, if the vanity is captured between two walls, measure, measure and then measure again. Determine if you will need fillers. Keep in mind that square 90 degree corners are rare so, even if the measurements say it should fit, an unsquare opening may prevent the assembly from fitting in reality. (Removing drywall from cabinet level down can save you sometimes.)

@Patricia Colwell i need to make my purchases now so that the contractor knows whether to leave the holes for the electric boxes next to the medicine cabinet for sconces or not (in case i use a long mirror instead and put an overhead light instead). so just trying to figure out the proportions ahead of time. a close member of my family thinks that putting a 36 wide medicine cabinet leaves 12 inches on either side of the medicine cabinet and that is too much. will a 6 inch wide sconce be better than a 4 inch? in that case it would be 3 inches between the medicine cabinet and the sconce thenthe 6 inch sconce and then another 3 inches. i think that would be balanced. this is the one i am considering that is 4.75 wide on bottom and 6.25 wide on top but its only 12 inches long and i was thinking that porportions may look better if it was longer: or this one that has a bit narrower but has more length 16t/8 and might give better portion: can you help me figure out the optimal proportions? should i be aiming for a 40 inch medicine cabinet instead? would that make for better proportions?