Blewers timber (local to Woofage) do oak mouldings at their Kent depot, but I'd be surprised if they do them that long.
Most of the oak we buy in as rough sawn is 3.2 - 3.6 metre lengths. If it's an odd size or shape bit you need then you may need to get a joinery firm to machine something up for you - CNC moulding machines are more common than you expect.