There are a lot of mfgrs. out there. One that I'm currently working with is Heritage Vinyl Products. They have local dealers. I'm in the process of getting sizing and estimates for posts, handrails, and balusters for our porch. We want a maintenance-free porch system. The only bad news is that their 'systems' come in 8' lengths. Ours are a little longer. The rep. is checking into getting 10' lengths and we'll have to 'waste' what we don't use (so it seems). Also, we have 8' posts and aren't installing the 'short' ones. Their systems don't accommodate a pre-manufactured 'link' between the handrail and the 8' post. So, I'll have to trace the profile of the handrail onto the post and route it out. No problem; just one more thing to-do.
A couple of other links are Discount Fence (Use the pull-down Menu in the top-left corner, select Porch, and press Go) and and Hoover Fence (From the 'list' on the left of the page, under Vinyl, select Vinyl railings.)