OP like a pp I'd seriously consider having a dress made for you.
We are going back nearly 20 years here 
but, like you I had a strong vision of what I wanted abs just couldn't find anything that was quite (or even nearly) right.
The colour was wrong, the neckline wasn't the right shape, the sleeves were too long/short, the skirt was too voluminous etc etc.
I just didn't want to compromise on something so expensive.
I paid £1500 for my dress but over half of that was for the super expensive silk fabric I chose.
The whole process was brilliant. I found a local dress designer/maker, described what I wanted and she did a few sketches for me until we nailed the design.
Then she made a toile (mock up in linen) and this was great as it allowed everything to be adjusted so it was perfect (for example I thought I'd wanted 3/4 sleeves but ended up with short ones once I'd seen the design on me) and I had the neckline changed.
Then choosing the fabric....any colour, any texture any weight of silk I wanted but was guided my my designer as to what would work best with the design (specifically over the weight of the silk). I bought that from a wholesalers and despite the cost it was a good price.
It was absolutely my perfect dress and I'd never have been able to buy it off the shelf.
It was super flattering as it fit me perfectly and looked far, far more expensive than what I'd paid for it (I think partly the fit and also because of the quality of the silk).
I'd definitely recommend you think about this option. You don't need to spend as much as I did.
The hardest part is finding someone talented to help design and make it but they are out there!