Check out http://webstrings.com/ they have some crazy cheep strings, but they might not be what you want.
www.bigcitystrings.com seems to have good prices, but I haven't dealt with them yet. They did return my e-mail quickly, though, when I asked them if they had some strings in stock. Some folks here recommend www.sheetmusicplus.com, but, again, I haven't tried them.
I currently use the Detroit Bass XLs from Webstrings on my Rickenbacker. They are nothing less than fantastic. I've used Fender, D'Addario and other strings in the past and all have been very good, but Webstrings.com is half the price and the quality is as good, or better than anything I've ever used.
i really like (and use) ken smith strings. good tension, great SS bright yet full tone, and not too expensive. cheaper than ernie ball or DR.
I love ken smiths and recently ordered some online from mf for $15 a set for five string, so they are not priced badly at all and last forever
www.juststrings.com has the biggest selection and lowest prices I have come across -- and I'm cheap! They also carry a huge selection of single strings, even Elixirs. And www.wwandbw.com has Vinci nickel roundwound strings for $9 for 4s and $13 for 6s. Hard to beat that.