I went to the spa dealer where I bought the spa. They only sold the complete lid, vinyl and all. I thought it was stupid to buy all the exterior stuff when all I needed was to replace the core. I searched around on the internet and found a place where I could have it shipped to the house...after the freight charges, I saved about $100. Then I tried to put the thing into the vinyl cover. That was like trying to put a 500 pound sausage into a 10 pound casing. It was hard work and didn't come out looking nice and "smooth" either. I got to thinking it wasn't worth the savings at that point.
Then, after the first winter with the new core, the lid got just as heavy as the 7 yr old one that I'd replaced in the fall. Turns out, the vinyl was shot in a couple of places and the core got nicked in a couple of spots (maybe torn from when I was trying to wedge the new core into it) which let the water in over the course of the winter. Long story short, I ended up going back to the spa dealer in the spring and buying the whole new set up. In the end, it only cost me a couple hundred bucks to have two covers in one year.
As my wife has pointed out to me since, I'm sure I did something wrong in the process to screw things up, but she wasn't willing to admit that the mistake might have been from a lack of supervision on her part.
Just my

worth of experience. Good luck.