I've used all kinds except for the push in type and I have found that these
horizontal poultry nipples are the best thing out there. This same ebay seller also has an
Automatic Fill Watering System that you could get, it is one of the nicest out there and the auto fill feature is something no one else offers. It is easily expandable as well and I do believe he sells the parts direct without having to go through ebay.
If you do a search for my user name, LoneOak, I have posted a couple topics about the different types of systems I have and am currently using along with pictures of most of it as well.
Just to add, I haven't seen any of these items at the local feed stores either. Even Tractor Supply doesn't carry them. I believe the stores have to buy such a large quantity to meet there return margins that they don't even mess with them. I asked at my local feed store and he said not even his suppliers carried any of the nipples or cups just the standard feeders and waters that they have been offering for years.