I second Bobbo's advice. That's exactly what I've done...in 4 different places (our house, 3 relatives we visit). Do it once & do it right! :)
I got mine from
RVPowerOutlet.com. And if you shop, you might be able to find it even cheaper.
It's nice having the 120V 20A AC outlet for tools, lights, radio, etc. while working outside. I use the 30A outlet to keep the rig plugged in during the winter so my 50A cord isn't exposed to the winter elements. During the Spring/Summer/Fall I connect to the 50A outlet.
~Rick
ON EDIT: I did some shopping for you. Here's something similar even cheaper, but I can't comment on the company.
50/30/20 RV Outlet from RVParkSupplies.com