Hmm thats not gonna be to difficult I found a schemtaic for you. If it is what I think and you have S-Video out put from your Laser disc and Coaxial input to your Television you can build it your self for about ~$3 I could check radio shack price lists if you want. here's the schematic for the signal combiner and seporator. so either way you have it here:
basically you can combine the seperated signals from the s-video with a capacitor. Or with an LC network. Try it out. No guarantees though as I have nothing that uses s-video to test it on.
Heres the page I got the info from: