I bought my counter pressure filler from Amazon (link below). At $385, it is not cheap but for the pressures needed with sparkling wine, this was the way to go. If you have a corny keg system, you are already halfway there. There are other counter pressure fillers usually used for beer that can be frustrating when when used at 40psi. I don't have a picture handy at the moment, will try to attach one later.
https://www.amazon.com/dp/B00N3D9POI/?tag=skimlinks_replacement-20
$3/bottle seems steep. My local homebrew store has them on $14/case and normally $20/case.
The benefit of the counter pressure system is that you force carbonate in the corny keg and then attach the filler to the keg for bottling. You have complete control over the pressure. You also do not dose with sugar, so no yeast sediment or exploding bottles from over-dosing. The idea is that you pre-pressurize the bottle to the same pressure as in the CO2 tank. Then to fill, you bleed out some of the pressure from the bottle and the wine is gently pushed into the bottle. Go slow enough and you have no foaming. Gotta cap it right away or you'll get a geyser. Takes about 40sec to fill a 750ml bottle.