Best bet is ebay. You will find ones for €40/€50 for PU ones and €80 upwards for carbon fibre ones. I picked one up for my is300h and just installed it there before the snow hit with automotive 3m tape (included). It was from Ikon Motorsports (some Taiwan brand)
Fitting is okay for the Taiwan branded spoiler. It wont be perfect and there is a millimeter gap at the end of each side of the roof spoiler but it doesnt really bother me. If you want something more flush then go with reputable Japanese brands but that will cost you a bit more.
This is like the one I got in case you were curious:
https://www.ebay.ie/itm/UNPAINTED-for-LEXUS-IS250-IS300h-SEDAN-4DR-D-TYPE-REAR-ROOF-SPOILER-WING-2015/182720503674?hash=item2a8afd9f7a:g:tPIAAOSwk1JWcM-V