There are many companies out there that make kits for both straight and spiral staircases. The WEB sites I've listed below do them both. (At least they say they do straight staircase / baluster kits.) If not, call or e-mail them. Be sure to give as much info as you possibly can so they can help you right directly. For example, what type of wood you have (or metal finish), stained or painted, etc., etc..
Stairs.Com, Stairways INC. I know you'll see that these guys deal in spiral stairs but, again, they 'tout' to deal in straight ones too.