Drain transmission fluid and refill?

Hi all !

I am owner of Vauxhall Crossland X 2018 automatic gearbox edition , and I trying to get ideas how to drain the transmission on this vehicle. I got the position of drain under the bottom and the filler plug already. 
 

but I got a difficulty to get where I can buy gasket replacement after unscrew drain.

i am reading Haynes manual and it says 

Raise the vehicle

Remove the level plug

 

Discard the gasket

Remove the overflow plug

 

Discard the gasket

Drain the automatic transmission

 

So I should have 2 gaskets to replace during drian the transmission fluid. 
 

I try type it in Google, find in euro car part, Halfords.

All I can get is just Engine oil drain plug replacement , I don’t think it would match ? It is ? 
 

look forward for answers, your help I will very appreciated