@Savage Clown
One bit of advice before you begin framing, since you mention it's been in a tube for a while you should take it out, let it breath for a few days then try to flatten it, the best route would be to cut an acid free foam cores from Michael's and sandwich the print in between it and let it flatten for a few weeks.
There are so many ways to go with framing and it really depends how much you want spend and how much you want to preserve the print. You can spend not a lot of money to hundreds of dollars on framing. Personally I've only ever framed my pieces professionally, I've used the bigger box stores cause independant framers are too out of my price range in my area though I say if you want to go that route you should check out local independent stores. We have a chain called Michael's that has a custom framing center. Most pieces I have framed ranged in the $275-$350 mark, but those are all 24x36 an 18x24 I expect would cost less.Those come with a lot to protect the piece.
Now if you want to go cheap you can get a cheaper frame. The stores that sell the custom framing, like michaels mentioned above, also usually have cheapr pre-made frames in fairly standard sizes. You want to make sure though to get acid free foam core backing, and try to use something like a photo corners (
Recollections™ PVC Photo Corners) so that way you are not putting tape on the piece itself. The cheap frames are cheap but it's something for a beginner or if you are not looking for something professional. If you have a store similar to a michael's you could see if their custom frame center could just sell you a cut colored mat, then buy a cheap frame slightly larger than the piece to make it a bit nicer.
Spotlight Displays (
Quality Movie Poster Frames for Any Size Movie Poster) is another option for you. These are decently nice frames but the benefit is after mounting them to the wall the hinges open up so you can rotate prints in the frame. The 18x24 frame is only about $55US. If you want one step up from a cheap frame but not quite ready to dip into real custom framing money this could be an option