If you want to get in without a big investment, try webstrings flats,
www.webstrings.com. I have been very happy with their strings and you cannot beat the price. labella's are awesome too, and I recently put TI flats on a 6 string and they are amazing, but very different from the webstrings and labellas, which are both high tension. But all of them sound good and get better as they get "played in." In fact, I am waiting on a set of webstrings flats right now (hopefully tomorrow) for my P/J bass. I do eventually want the TI's for it though because I am digging the low tension. Got my TI strings at bassguitarstrings.us , their service was great and the prices are the best I have found yet, although the TI's are expensive no matter where you go!