If you want to duplicate your existing manifold because its rusted or something, try Ah Heng USJ. May be costly but workmanship is superb. Make sure you ask for him to personally make it though.
If you want custom aftermarket ones to increase your performance, well better not... making a good manifold is some proper science. Or else you can try googling Speedworks/Hotbits, they do produce for many odd cars based on... "references"... on other manifold makes.
If you dont have a manifold at all to begin with because you're restoring an old car, I'd still recommmend talking to Speedworks. Or maybe Powerzone. They seem to know their stuff...