Hi @ScottW,
Thank you for your question and welcome to the Bunnings Workshop community, it is fantastic to have you join us.
I've spoken to our suppliers who've unfortunately informed me that it isn't possible to order downlight diffusers as a spare part.
My suggestion is to pop the downlight out of the hole and see if it is a quick connect style downlight or if it is hardwired. If it is a quick connect, you will see a power plug and a power point in your ceiling.
If you remove the existing downlight and find that it is hard wired, you will need to get an electrician to swap them over for you. If your downlight is a quick connect, it would be very simple for you to unplug the old one and replace it with a new one.
If you find that it is a quick connect, you could purchase a Arlec 9W Tri-Colour Dimmable LED Downlight and install it yourself. This is both a quick connect downlight and has 3 options for the colour of light that it outputs so you can match the existing downlights.
You might like to check out these previous discussions for some further information, Replacing existing LED downlight, How to replace LED downlights?
Let me know if you have any further questions.
Jacob
