Can't find this standard ceiling plate anywhere

[bulgin]

Hello. I hope everyone is safe and healthy.

Please see the attached photos for reference. I have a ceiling with a junction box or maybe it should be called a "junction circle/box" - photo #1. It is 3 3/4" in diameter. This is where I want to mate my fixture displayed in photo #2 which is 5" in diameter. I need to mate these two together but all the of the junction circle/boxes (Lowes and home Depot here in the U.S.) I see are not 5" in diameter. As well, as you can see in photo #3 (which is only here as an example and not the right size) I need something like that with the tabs on the side that bend down so as to connect to light fixture in photo #2.

I am in the U.S. and any advice on what this is called in electronic terms and any possible places to purchase such a thing would be greatly appreciated.

 

[littlespark]

Be only ever seen one light fitting that came with a circular mounting plate like that…. But I didn’t measure it, nor remember where it came from.

The majority of decorative light fittings here in Uk are just a flat metal bar bracket with the fixing tags bent down at each end. They should come with the fitting, along with the small screws to attach fitting to bracket.