Wad depends on powder.
If I'm using a bulky magnum powder for max velocity (Bluedot, Longshot) I use WAA12R's or the Claybuster equivalent. They allow for more room for powder in the hull.
If I'm loading moderate recoil slugs I use WT12 or the Claybuster equivalent. They fill more of the case when using less powder or a less bulky powder (Unique, 800X).
I buy my wads locally so I'm no help in ordering online. Both seem fairly common though.
Both can be ordered through Bilozir from Ballistic Products but that can take a while.
For hulls I picked up a few hundred Federal 00 buck hulls when the police were out training one day. Remington high brass are pretty much identical in terms of internal dimensions. Winchester high brass have thicker base wads so have less internal volume.
I always put a 20ga .125" card wad under the slug inside the cup. I found I had a lot of flyers and pinched off pedals from the wads. Doing this really helped with accuracy. I order those from Track of the Wolf in the US.