De cat link pipe

Could you guys point me in the direction of the best place to buy the link pipe to remove cat and flapper valve , but using stock headers and silencer on a 2014 hp4 carbon.
Bought a bike with a short deecated Austin racing exhaust fitted, which will be for sale

Ive just tried to order the decay part from rainbow BMW but they refused to sell it without the full race kit?? he stated that the feeling would be off? i can't see this as the lambda sensors are before the cat.

I rang williams in manchester gave them the part number and order placed. Bmw uk have some in stock. the part number is 77118521688.
you can also get the part from alpha racing but with delivery it makes it about ?30 more expensive.