At 550 you *may* be able to qualify for a store card but even then most want a 600 or better.
Unfortunately, a secured card is the BEST way to go as it gets you a reputable, national bank that you can grow with. Many banks offer such cards; BofA, Citibank, Wells Fargo, US Bank, PNC etc. Obviously, if any of these creditors were burned by you in the past, skip it.
Your other alternative is to get an "unsecured" card from First Premier, Credit One (not to be confused with Capital One) or Applied Bank. These banks should really be your last option. Although unsecured they charge ridiculous fees that essentially equate to what you would have put in the bank as a security deposit; only you never get it back. Also, check terms and conditions because last I checked Credit One and maybe even Applied Bank did not offer a grace period, so the minute you swipe the card for a purchase, you start paying interest.
There are tons of credit forums out there. I suggest looking at the My Fico forums and reading about rebuilding there. Also http://www.whoegavemecredit.com
has people that report whether or not they were approved for credit from a certain lender with certain scores. Of course, take anything you see there with a grain of salt...some of the dates are also old (2004) and creditors have become more strict after the financial crisis.
You should look into getting a real Fico score from all three lenders to see which has your highest score and then look for lenders that only pull from that credit bureau. And remember that a FICO score is only one thing that the banks look at. If you have recent lates or charge offs that will not bode well for anything other than secured cards or bottom-feeder cards (First Premier, Applied Bank and Credit One).