Try The Wooden Hill Co - they are one of the few companies I know of who make "DIY kits" and they are reasonably priced. If you want an all wood I think you'll struggle. Last one I put in was specially imported from Sweden by the householder (it then had to be "narrowed" as the builder made the opening too small!). You'll have to make sure that the floor beneath the central column is strengthened (unless you're going straight onto a 24 x 4 beam) as these systems rely on having a relatively small plate at the bottom which carries ALL the weight of the stairs and the person climbing them. Similarly at the top they need a strengthened anchor point, so you may need to double-up or strengthen the joist there as well depending on what you've got.
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