I live in Vientiane and haven't been to Thakhek yet. But from my experience finding accommodation in Laos is both easy and cheap, especially when you get outside the larger cities. But the farther you get to the country side, the more primitive things will be of course
There is always the classifieds in the local newspapers to look for accommodation (but as they are mostly in Lao, they may not be of much help to you). Instead you may try to ask the receptionist in your hotel, to set you up with an agent. They will usually know someone who knows someone who is an agent...
A few hundred dollars a month will bring you far in this part of the world