The reason you won't find lots of adapters that plug directly into the socket is because it's a bit hard to make reliable adapters capable of providing so much power in such a small package that's also light enough to not affect the socket you plug it in mechanically.... you don't want the weight of the adapter pulling the adapter out or bending the internal contacts of the socket.
These adapters are generally sealed, so that no humidity and air will get inside (to protect people and house against shorts and other things). This however has a side effect... the heat generated inside by losses in power conversion has a very hard time getting out of the box... the air and components inside will slowly heat up and the only way is through the mains leads and the plastic of the case (poor conductor of heat).
With an efficiency of about 85-90%, so you can imagine that if the adapter produces 50 watts of power, about 5-10 watts of power are wasted as heat. A small adapter like the ones that go straight in the mains socket aren't really capable of making those 5-10w of power go outside the case so eventually the adapters will fail.
I may be wrong, but I personally wouldn't trust an adapter that goes straight in the mains socket and advertises more than 20 watts output power.
Quality adapters in the style of those brick laptop adapters have more room for a circuit board inside which allows for more complex circuits which in turn can be much more efficient (some reaching 95-97%) so they don't produce as much heat inside even though they're capable of generating more power. So for example, if you buy an adapter rated for 12v 6a (72w) it will work just fine if your lamp uses only 50w and shouldn't be that warm since it will not used to its maximum capability.
But again, it's important to be brand name, quality adapters, there's lot of fake Chinese laptop adapters out there that aren't even capable of what they advertise.
You could simply take the power cord and tie it to the plastic case of the adapter reducing its length, and then the adapter will simply hang between the socket and the floor... it will look ugly maybe but it would work.
Here's some suggestions of quality adapters from Digikey (a reliable electronics store that doesn't sell fakes), you have there adapters that output 12v and 45-100w :
http://www.digikey.com/product-search/en?FV=fff40009%2Cfff8004a%2C8000d%2C8000e%2C80052%2C80056%2C80061%2C80078%2C80216%2C80285%2C803a5%2C8043c%2C804af%2C806af%2Cc00002&mnonly=0&newproducts=0&ColumnSort=1000011&page=1&stock=1&pbfree=0&rohs=0&quantity=10&ptm=0&fid=0&pageSize=25