Use your favorite WEB Search Engine and use keywords like 'vinyl' and 'porch' and 'rail' and/or so on. Heritage Vinyl Products, among others, has what you want. As a matter of fact, at my wife's insistance, this is my Spring project. I'm replacing wood, decorative posts, rails, and balusters with vinyl-based products. IF you're looking for structural posts, where they hold up a porch, you may be hard-pressed to find them in vinyl. Obviously, vinyl isn't meant to support the weight of a roof. Just something to consider.
You didn't say if you're doing a deck or a porch. The deck material is more available than the porch material especially when it comes to 8' high posts. Also, many systems are sold in 8' lengths. Take you time and search around. If you need more specifics or are having any trouble, e-mail me directly.