You might want to try a Brass Store. That was the only store that had the type of brass door-stop we needed when we had our hardwood floor installed. (The ones at the Home Centers were too low. Our 'threshold' is actually about 3/4" ABOVE the floor!)
Anyways, if they can't help you, ask them who might. YOu can try a few Hardwood Floor Retailers (found in the YELLOW PAGES.) Or, some carpet stores. As an aside, I'd like to suggest that you get that 'rubber backing' that protects your hardwood from getting scratched by the bottom of the carpet. The SLIGHTEST back-and-forth movement from carpet on an UNPROTECTED hardwood floor will wear out the finish AND eventually the wood if there's no protection between the 2. There are standard 3' and 5' runners in stores like K-Mart (where we got ours) that have this type of backing pre-sewn on the runners. Check them out to see what I'm talking about. It's, basically, a no-skid backing. Now, I know the runner you need is going to have to be a WHOLE lot longer than 5'. No problem! As I mentioned, you can buy this rubberized-backing as a STAND-ALONE product at K-Mart. (Wal-Mart or even Home Depot may have it too.) You can custom-cut the backing to fit your needs. I guess you could cut it 1/2" narrower than your runner so you don't see it. (This will leave 1/4" on each side.)
Sorry, I don't have more to offer. No matter how you end up installing your carpet, highly consider the protective, rubberized backing. My best to ya and hope this helps.
Jay J -Moderator
PS: If you're buying floor runners and indoor floor mats too, I suggest you buy the ones with the backing already pre-sewn in them. If you create your own, say for example, 8' runners where you cut your OWN protective backing, experience has shown me that since the backing isn't pre-sewn on, the runner/mat tends to 'slide off' of the protective mat over time. It would be one thing if you were doing this for a much larger piece of carpet, for say like an area rug where furniture would help to 'hold' the rug and protective backing in place.