Where can I buy knot-free skirting boards, door linings, cill boards and architraves in soft wood, or is this not posible?

Am planning to varnish/stain it all to a similar medium oak colour to match the floor, without going to the extent of buying solid oak items (cost is one reason!). Plus I want it to match the stair handrail already fixed in sortwood.

I dont fancy a whole array of knots visible as you get from the typical gear from B&Q etc.

Any ideas, cheap sources in SE England/London or guidance?