Hi cokewithvanilla,
The first place I would start is reading this article.
As you will see every mattress has two basic functions which is pressure relief (which is the primary job of the upper layers of the mattress) and support (which is primarily the job of the deeper layers).
Shoulder pain generally indicates an issue with pressure relief and is fairly common with men (with side sleeping) because they have wider shoulders than women which need to sink in a little deeper so that the torso makes firmer contact with the mattress and can take the weight off the shoulders. If this is the case … then a suitable topper will generally solve the issue. Post #8 here has some guidelines about choosing a topper.
If you do decide to go with a new mattress (although I would suggest trying a topper first) … then post #1 here and the information it links to will give you the basic information you need to have the best odds of success. I would also suggest reading this article before you consider another airbed (although now that you have one I would probably try to make the best of it).
Hope this helps.
Phoenix