I just bought a new cover for my Supra and here are my thoughts....
You can get a stock cover from your dealer for $550 that fits well, but it is made of cotton/poly and doesn't hold up very well.
You can have a custom cover made from Rankin, Evolution, or similar that will fit great and last for years. However, they cost between $700-$800. These places typically use Sunbrella.
Lastly, you can get one from Westland through Overton's or elsewhere for about $350-$400 made out of Sharkskin. It's better than cotton/poly, but not as good as Sunbrella. This is what I just did and I'm not that happy with it. I had to send it back because the cut-outs for the tower legs didn't line up right.
I just got the replacement back and it fits better, but still not as well as my original stock cover. No matter how tight I get it I still can't get the area over the bow taught. Even with a pole holding it up I'm afraid that there will be some pooling.
So, long story short, you get what you pay for.