Hope its sorted. If you have the original part number of something its always a good idea to google just the part number. This usually throws up loads of suppliers. Unfortunately many are overseas, and since brexit fewer are willing to ship to the uk.
But as others have pointed out there are probably many aftermarket alternatives, or alternative fixing methods. There are lots of similar of plastic plugs used for attaching other body trim items. Many cars use a type that is basically similar to this, but are single use. But cheap to replace. An expanding plug goes into a hole in the bodywork. Then a single use pan headed fixing is driven in, (rather than screwed in) holding the tray, and expanding the plug. General garages will often break or lose things like this . They wont want to obtain Honda originals every time.
My Yaris has a flap in the undertray so you can replace the oil filter without removing the tray
As to the aerodynamics of an undertray. The benefits are greater with lower ground clearance. And the trend now is also towards lower ground clearance for the front bumper/skirt. This too can make a big difference to aerodynamics and fuel consumption.
The aero type enclosed wheels increasingly used on Ev's also makes a difference. Tesla claim a 3-4 % improvement in range. Not sure if the windmill type wheels on some mk 4's are this effective.
