You might try rejuvenation.com or restorationhardware.com online. Both are good sources for restoration projects. Rejuvenation has a Live Chat resourse you can use and if these don't have what you're looking for, I would think they would be a good resourse for a referral to point you in the right direction.
You can usually find wooden finials at local home/hardware stores and some craft stores. (often used as feet for furniture) These could be painted any color, or varnished...if you are looking for a cheap solution.
Also had a friend use new tennis balls, cover with a pretty, stretchy fabric and stuff the ends of the fabric into a hole in the ball. Then screw the ball onto the bed post.