The Hangar-9 Solo is basically a text-book trainer design... the measurements are darned close to those of many, many very good trainers. You may be able to find either a wing or wing plan that's readily available for another similar trainer -- and if not, I have no doubt that you'd be able to find a plan for one that will fit your fuselage just about perfectly.

Your Solo wing should be 62" span and 11-1/2" chord, flat-bottomed airfoil with ailerons and a couple degrees of dihedral. Any flat-bottomed 4-channel trainer wing with about an 11 to 11-1/2" chord should fit your plane -- and as long as it's got about the same dihedral as the Solo, it will fly very much the same.

Some other planes from which you could try for wings and/or plans are the Tower Hobbies Tower Trainer 40, the Hobbico Superstar 40, or the Great Planes PT-40. Most of the manufacturers do sell plans for their kit planes (maybe not for ARFs, but usually yes for kits) -- and if the manufacturers don't come through, the PT-40 was originally published in RC Modeler and plans for it are still available from them.