If you do get a supply of them... unlikely to get a large quantity from a wholesaler if they even have them listed; I would suggest to replace as part of any maintenance.
The transparent wiring cover will easily flick out at the hinge with correct application of a flat screwdriver, so you could have covers ready to go with spare wiring covers (buy a few dummies).
I know it's not the point but you could consider getting some adhesive 12v LED tape instead, most thinner stuff can be cut at 8-10cm sections make them up ready to go.
It's less hassle to go forward using the same box if you want, but not offering or connecting the illumination.
As I say I don't offer illuminated sounders every now and then a new customer enquires about them, I ask if they have or have applied for planning permission to have one and then explain the details of this.
If the customer even wanted to put the effort in to apply, your unlikely to get it granted in a domestic situation.
No neighbour wants to look at your light up advert looking across the road in the evening... more importantly, I don't feel they should have to either.