There is a wide variety of lowers you can get. I'm not aware of any Colt lowers for sale new. What you MIGHT find, is someone who, many years ago, scrapped their Colt Lower to say......PUT THE SLIDE ON A PARA FRAME, and still has their old frame lying around. Talk to your gunsmiths in the area or look on gunbroker.
Go to brownells.com, and order their free 1911 catalog. You will see frames from manufacturers like Caspian, a lot of options. I PERSONALLY am not at all loyal to Colt in particular. I have owned them, I saw nothing special in them compared to other 1911 mfrs. The thing is, you won't stop at one. If you put that slide back on the single-stack frame, you will have that wide frame lying around, and since you have converted one, you will now see that it's not that big of a deal to convert another. Then you will want a frame to put a .22 conversion kit on. Now you're in trouble. It never ends.
I have had a Para, at the time I thought the higher capacity was nice, but as I learned to shoot more, I realized that in reality, the likelihood of needing all 13 rounds without a pause is small indeed. It is also bordering into trying to make a pistol do a rifle's job. I now carry a single-stack .45 and I don't miss any of the staggered mag guns I used to carry at all. Even if you DO have the higher capacity, you still need to train to change mags on the fly either because of a malfunction or for a tactical reload, (Changing mags when you CAN, rather than when you HAVE to,) and the higher capacity loses relevance.