Advice regarding the outflow pipe bits please

[Lovethekoi]

Hello all,
We have plants in the higher pond and fish in the lower one. The water flows from the higher to the lower as seen in the picture.

Where and what can I buy to replace the 'half pipes' type things seen here? Thanks

 

[Lovethekoi]

 

[GBBUDD]

Not an easy task but it's not terrible either. If I had this to work with I would only want to do minimal disturbance to all the algae it seems the wall bleeds or water just leaks over the top either way I like it sooooo. I would leave everything the way it is But I would either come up with a design where I just built concrete or stone walls around the half pipe then just hide in with again rocks and more concrete or make the concrete look like rocks. So when you look the pipe is hid but it is still the waterproof layer it's just hidden