Future as it was called back in the day, is just an acrylic floor wax. You should be able to find at dollar stores under the LA brand. Dollar General has their own brand and it has marked on bottle says, compare to future. Many brands out there. As long as it is a clear acrylic product, you should be in good shape. I coat my old decal sheets with either a Clear Acrylic Spray, or from Micro Sol chemicals they have a decal overcoat. But you could use the future as a clearcoat over the decals and finish. It can be sprayed through an airbrush, using windshield washer solvent "the blue stuff" as the thinner with no tinting from the blue in the solvent.
Refer to thread http://www.modelcarsmag.com/forums/index.php?showtopic=86596&hl= Mr Chapman gives a great tutorial with future as a clearcoat and thinning with washer solvents.
Here is a pic of the Family Dollar store brand