These are really cool figures and have a slew of news ones coming out. Where, besides target, can these figures be bought? I almost never find them. And the internet will charge you a ton of money and your first born.
Don't know if you have gotten any replies yet or not regarding your question about where to buy Justice League Unlimited figures from so I thought I would post an answer. Keep in mind that what I mention is an answer will only apply if you live in America. Outside of America I have no clue where to get JLU figures aside from the internet.
JLU figures are supposed to be only sold at retail at Target stores. Mattel stated a while back that the JLU toy line was Target exclusive. However, you can also find some JLU figures at Odd/Big Lots stores and Burlington Coat Factory stores as well. Many discount stores such as Family Dollar or Dollar General seem to get older JLU figures in from time to time.
Other than what I have said above, the internet (of course) has a ton of websites that sell JLU although the prices are typically outrageous. One site to look at for Mattel exclusive JLU figures is Mattycollector.com which is the company that produces the JLU figures. From time to time the website will have an exclusive JLU figure for sale that can not be found anywhere else. For example, the next exclusive JLU figure to be sold on the Mattycollector website is a LOBO figure. It will be sold in June. If you want to see pictures or more info go to the website and check it out. The Lobo figure looks incredible if you ask me and I will definitely own one when they become available.
I hope this helped you out some if at all. As I said, everything that I mentioned was pretty basic and general information but there are sources online that offer much more comprehensive info on JLU. Just look for it and you'll more than likely find what you are looking for. Send me a personal message if you need more specific information on JLU and I'll try and help you.