Look for a reputable local cabinetmaker. Any kitchen base cabinet can also be a bathroom vanity cabinet, and a lot of them make bathroom-specific products as well. This will be more affordable than getting built in place custom.

Not necessarily. A Google search will show you there are 67 inch double sink vanities out there. Unfortunately for me, I ended up happier with the double sink vanity I purchased from a big box store than the more expensive one I had custom made.

IMO 72" is the min. for 2 sinks and honestly I prefer more counter than sinks. We have been married for almost 60 yrs and have never had 2 sinks and never had a toilet room either .Post a to scale plan of the bathroom maybe we can help a but more. I do agree with Mark this is easy to do without going truly custom.