can i get an unlocked iphone without service and just use a sim card.?

Well, what are you going to do with it without service? AT&T is the only one (to my knowledge) who supports the iPhones. Not only that, but I used to work at an AT&T call center and we would have people call in all the time with phones they had unlocked from different carriers and sometimes they didn't work right. So you're going to have to get service from AT&T most likely to get the thing to work, unless you just want an MP3 player, and if that's the case you might as well just get a new one from AT&T.. unless you just don't want to pay full price for a new one.

Still.. I'd be surprised if everything you wanted to do worked out ok *and* the phone worked with no problems.