tounge and groove MDF panels for staircases

Does anyone know where I can buy tounge and groove MDF panels for staircases. The sheds have panels suitable for horizontal runs but obviously with the stairs having a pitch of 42 degrees I wouldnt be able to cut the required shape out of the panel.
I do have a router and could make my own but if its already available then it will save me some time!
Anyone have amy links of UK suppliers?
EC .

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