My bathroom vanity is custom

The pros are that aesthetically it is exactly what I wanted - wood species; stain color; design of doors

Functionally it is also exactly the way I wanted it to be as it all contains very specific configuration and the drawers and pullouts were designed to be exactly how I wanted them to be.

I guess the con would be the cost potentially although the cost of my custom was probably no more than high quality ones that had upgraded interior features.

Another con is the stress of designing something when it can theoretically be exactly what you want :-)

Where can I buy pre made vanity online?

Many places.... to name a few:

• Here (Houzz)
• Wayfair
• The Home Depot
• RH

Prices and quality vary drastically.... read reviews carefully.

What are pros/cons of using premade vs built

Biggest con for premade is size... e.g. if you were hoping to fully fill your vanity-alcove that has a 53.5" width - it will be practically impossible to find something pre-made in that exact size, let alone something that you like.

If you are flexible with size and some finish choices - pre-made may be less of a headache (as you wouldn't need to coordinate too many things - cabinets, top, sink, pulls/knobs, faucet, ...)