The belt mounted shell holder in that YouTube video of PFC Horner and SFC Johnson is made by Progressive Machine and Tool. It is a nice piece of "kit". It is made out of milled aluminum. I also have the California Competition Works shell holders, but they are plastic <sigh>.
As far as the actual speed loaders go, the plastic tube-y thing-a-ma-bobs, well, I never used them, but in all the 3 gun competition videos I have seen, it looks like it helps if you have the Aredondo mag well attached to your shotgun.
EEwww...Wow! They aren't cheap.

go here:
http://shootersconnectionstore.com/Arredondo-Speed-Load-Assist-Benelli-P71.aspxThey want $154 for that hunk of aluminum.
Then there are the Saiga shotguns. They take a box magazine and look the whole thing looks like an AK47.