There's already been a lot of good advice written here, so I'll try not to repeat too much. Apologies if I do.
So, like many of the other people who have responded, I do not play the NSP myself--yet. I've played Scottish GHB and SSP since the mid-90s and uilleann pipes for about 20 years (with some extended sabbaticals from each at various points). I play a set of SSP in A by Hamish Moore, which I love, but in recent years I've really been drawn to SSP in C, so about eight months ago, I started looking for a set. The price had certainly gone up quite a bit since getting my set from Hamish in 1996 for about £900. (Which I remember seemed like a fortune at the time! Ha!) I really wasn't sure I wanted to throw well over $2000 at an instrument that would likely have limited use playing with most of the other musicians I currently know. Then I got into an interesting chat with a fellow who plays SSP and NSP who said that he just plugs a C SSP chanter into his NSP, tunes three of his four drones C-G-C, and away he goes.
I've loved listening to NSP since very early on, and over the years, I've learned a few Northumbrian tunes that fit the Scottish scale or that can be played in G on uilleann pipes, but I'd never seriously considered taking them up before. This discovery got the bagpipe-acquisition-gears cranking in my brain again, and after contacting a few NSP makers, I ordered an 11-key set in concert F with a matching SSP chanter in C from Philip Gruar. I'm hoping he'll be done with it later this year, but I'm not in a huge rush. While certainly not cheap, the total cost is quite reasonable and not that much more than I would have paid for a set of SSPs from a good maker.
I decided on concert F pitch, mostly because I just like the way they sound the best (concert G pipes in particular can be a bit shrill in my opinion), but if I really get into playing NSP I might order an additional 7-key chanter in "traditional" F+ pitch since it seems that many pipers both in the UK and N. America default to this pitch when playing together.
One thing I've done in the meantime is get a Fagerstrom techno-chanter set up for NSP pitch and fingering. This is an electronic chanter that has drones you can turn on and off. They retail for about $350, I think. If you're interested in NSP but not quite sure it's right for you, this would be a great way to try them out without breaking the bank. I got mine from Pipeline Bagpipes in New Jersey. If you order one, make sure you specify that you want the NSP version, as there are also GHB/SSP-only versions. (Mine does all three, which is kinda nice.)
http://www.pipeline-bagpipes.com/PCT/pr ... Pipes.html
You should also get in touch with the good people at the Northumbrian Pipers' Society, who I've found to be very helpful and encouraging.
https://www.northumbrianpipers.org.uk/
If you can stomach Facebook, the Northumbrian Piping Newsgroup is actually a really wonderful bunch of people, and many of them have a delightfully wicked sense of humor. I think I just saw a post from someone in Vancouver, BC selling a Burleigh set just the other day.
https://www.facebook.com/groups/131491660229952
As for NSP makers, Philip Gruar doesn't have a webpage, but his contact information is on the Northumbrian Pipers' Society website. Other good makers worth contacting would be Kim Bull and Paul Tabbush. Andy May makes great sets, but I think he said that his current waitlist is 3+ years. Ray Sloan's NSPs have a great reputation, but they're also a bit more expensive. Some other well-known makers like David Burleigh, Richard & Anita Evans, and Dave Shaw are retired or at least semi-retired and not taking new orders. I think Mike Sharp in California mostly focuses on repairing sets but it wouldn't hurt to ask. John Liestman was for many years pretty much the only pipemaker in the US who focused on NSPs, but I don't think he's taking orders anymore. (Though again, it wouldn't hurt to reach out and ask.)
Used sets, particularly Burleigh sets, appear for sale online from time to time. I think he made something like over 2,000 sets, and the consensus seems to be that they're generally pretty good, though some are better than others. Even if they're not in the best shape when you buy them, there are some pipemakers, like Mike Sharp, who could get them in good working order and maybe even improve them.
Lastly, there are a whole lot of great Northumbrian pipers out there to listen to these days for inspiration. There's a lot of great music on Youtube and Soundcloud at the moment if you take the time to look for it. In addition to "big" names like Kathryn Tickell, Chris Ormston (probably my fave), and Andy May, there are a whole lot of great players not that well-known outside the UK. Edric Ellis, Anthony Robb, Alice Robinson, Adrian Schofield, and U.S.-based pipers Bill Wakefield, Ian Lawther, and John Dally are all well worth a listen.
Good luck! Keep us posted on how you get on!