If all else fails,
http://www.vtigunparts.com/ sells original factory gun parts, including nipples. You'd need to know who manufactured your 1851 Navy. Then you should be able to find it on the VTI site and buy the exact original replacements for it.
Unfortunately, if replacement brands have advantages over the originals you won't get those advantages this way. Also, VTI seems kind of pricey for nipples. But the originals on my own revolvers seem to work well and last, so perhaps this isn't so much of a problem.
Wouldn't hurt to phone the companies Arcticap suggested, tell them what brand you have, and see what they have to say.
I've only browsed catalogs so far, but it looks to me that it's likely several different sizes or brands of nipples might work for your revolver-- those nipples having been designed for different advantages. There will be the original style direct replacements such as you'd get from VTI. I've seen some that say they're set up for a #11 cap-- my original Piettas seem to work best with a #10 Remington cap, but the local store doesn't carry them, so a different set of nipples for a different sized cap would be logical. Then there are different designs. One I saw touting their smaller flash holes, which are supposed to reduce the backward flash after the cap fires, thus reducing problems with split or shattered caps and perhaps reducing the chance of chainfires. Then there are stainless steel or...
Sorry. I did have a point. Which is that aside from there just being different companies each offering replacement nipples, there are also different designs of nipple, and all designed to fit the same gun. So it is possible the two different sizes you saw listed really were both for your gun.
On the other hand, I have also seen nipples offered for original Colt 1851s, and I gather the thread size on those puppies is different from what the modern Italian replicas use. So you have to be sure the part is designed for your particular gun.
Stupid side story: Many years ago I bought a matched serial number set of Ubertis. One was an 1849 Pocket Model replica, .31, the other a Second Model Dragoon. I bought them from Cabela's, with their "starter kits."
Among other things, the starter kits each included a nipple wrench and six spare nipples. In spite of the fact that the .31 was a five-shooter. These nipples would not have fitted the guns they came with. The .31 used a much smaller nipple, and the nipple for the Dragoon was a little bigger.
I had bad luck with those revolvers. I think mainly because I hadn't read this forum-- indeed, this was so far back into the dark ages that we got our information from baked clay tablets written in the Babylonian alphabet. The tablets were circular, and a store of information would be a stack of these clay discs. Every so often you'd knock them off the table, and that was the first hard disc crash.
Um.. point.. ah, my point was this forum probably didn't exist back then. So I didn't know how to use the revolvers properly. Had problems with them and sold them. I kept the "starter kits" and the nipples though.
More recently I called Cabela's and ordered their pocket .31 Remington, and then the .44 New Model Army Remington, both manufactured by Pietta. The .31 Pocket uses a smaller, shorter nipple of its own design. But the .44's look like an exact match for the old nipples from those Cabela's starter kits.
I think I have enough spare nipples to keep that .44 going for a long, LONG time.