Radius curves
Building a raidius curve will not be an easy task. Purchasing one that has be already preformed for you would sound like my best suggestion.. Yes it will cost as much as two sections of material, but creating one yourself will take you much time and maybe even equal expense.
In regards to making your own, what type of materials are you using ?
Aluminum, wood, vinyl?
In aluminum I would for sure suggest purchasing your materials custom made to order.
PVC... I would suggest the same but you may be able to create your radius yourself with heating the back-rails and bending while being heated. This can not be accomplished if you are using a picket that is inserted through the rails from top to bottom. I would assume this can only be accomplished if you have the type of picket that is attached to the face of the back rail.
In wood, well there are options. Please keep in mind this is not my field. What you read below are just suggestions to maybe get you on a path of thinking up a better idea on your own.
One option would be to soak the back rail for several days submerged in water. Warm water may be better. Then bending the back rail to your desired radius. I would attach the rails as they are still wet soaked through. They may dry in place. I would think there would be shrinkage while drying . Allow for such.
Another way would be to build a back rail out of plywood strips all glued together. All being bent to your radius then glued to each other. I have seen this in furniture. But that furniture was an indoor object. Plywood may or more than likely will separate when exposed to the weather.
After you get your bowed back rail, then I would attach your pickets.
Still , overall I would suggest ordering your section of material through a custom wood shop.
I am sorry I could not be of more help.
Good luck