I've been trying to find cheap but good AC wall adapter with good quality Tip (5.5 x 2.1 mm, I'm using these power connectors) for a while unsuccessfully so I'm hoping someone did legwork and can recommend me where to source it. I bought bunch of $3-4 switching supplies from eBay and all of them have Tip that breaks contact with DC Barrel socket far to easily. Slight movement of the cable and my clock looses power. Additionally they all come with short cord (about 1m).
I have some AC adapters around house from external hard drives and they seem to have really good Tips, which stay in the socket even if I yank them slightly. I noticed physically tips in better ones have some kind of split inside... Not sure what it's called.
Here's one of them I was able to find on Amazon. It's perfect for what I need but it's $8 a piece and they only have few in stock. I need to buy 10-15 for under $4 each. So if you know where I can find some let me know please 
Specifically I need switching 6Watts adapters w/ DC voltage from 6-12 (US socket and 110VAC), UL certified would also be great!
Your problem might be that some of your adapters are terminated with 5.5 x 2.3 mm DC plugs rather then 5.5 x 2.1 mm DC plugs. This results in a poor/intermitten connection from the 2.1 pin in the jack making contact with the 2.3 inner connection of the plug.
Lefty
The price is outside of your budget but I like this one...
retrolefty:
Your problem might be that some of your adapters are terminated with 5.5 x 2.3 mm DC plugs rather then 5.5 x 2.1 mm DC plugs. This results in a poor/intermitten connection from the 2.1 pin in the jack making contact with the 2.3 inner connection of the plug.Lefty
That's most likely the issue 
Since there's no 2.3 mm standard, it's probably poor manufacturing...
Thanks! So you had positive experience with these? Your link actually is for 5V supply, but I looked at the manufacturer and found they make these in other voltages (i.e. EPS060100-P5RP CUI Inc. | Power Supplies - External/Internal (Off-Board) | DigiKey)... If I can't find anything cheaper I'll go with these, thanks! Great cord length (1.83m) btw!
bratan:
Thanks! So you had positive experience with these?
I have. I use a few products from CUI. I am very pleased with them.
Your link actually is for 5V supply, but I looked at the manufacturer and found they make these in other voltages...
Other voltages and other terminations (pigtail, straight, right-angle, connector size). The datasheet lists all the options. As far as I can tell, only a very few resellers stock CUI products. If DigiKey does not have exactly what you want you may have to contact CUI for a list of resellers.
If I can't find anything cheaper I'll go with these, thanks!
You are welcome.
Great cord length (1.83m) btw!
Shape, size, weight, cord length, good electrical specifications, and a vendor I like. It's all good!