I'm just back from a year teaching in Taiwan, and will be heading to mainland China for another 6-8 month teaching period shortly. You certainly can find teaching contracts for 6 month periods, but it will limit the range and quality of schools that you can teach at. Also, if you are really set on Japan, which is supposed to be a great place to teach, getting into a good teaching program there now often requires a two year term.
Japan and Korea seem to be the two countries that pay the best out there, followed by Taiwan, and then followed by Thailand and mainland China (pay in Beijing or Shanghai is pretty good, but dramatically lower elsewhere in China).
I wouldn't recommend using an agency. There are plenty of agencies out there which are basically set up to rip you off, and you can quite easily find a job out there by yourself. A good place to start digging up information on schools, job offers and teaching experiences in different countries is
www.daveseslcafe.com .
Honestly, I would recommend starting to look for where you want to go, and looking for job offers early, so that you can really have a good pick of things before you get there, and have a little more of a feel for what the job market will be like. I also think that you should be open to the possibility of switching schools once you get there though, as when you're on the ground in the country is when you'll really see how many opportunities are out there and you will have the best first-hand info on what schools are good to work for.
If you've got the time, I would really look at doing a full year contract, cause I can pretty much garantee you that once you're living there 6 months will fly by and will not feel like enough. Asia is an amazing place, that you could spend a lifetime exploring, and you'll probably want a little time for travel outside of the country you're working in.
Anyways, if you want any info you can PM me. I have lots of friends teaching in a variety of countries around Asia.