I only have a single flash so buying a pair of 622N for use as receivers seems like a waste. What are some alternative RF receivers? Do I get to keep i-TTL and high-speed sync if I use a non-622N receiver?
The cheap compatible receiver for the YN-622N-TX is the YN-622N.
You can buy a single YN-622N transceiver if you want. And there may come a time when you actually want to have two lights off camera. In addition to this, if you have two YN-622N transceivers, you don't actually need a YN-622N-TX transmitter, since a YN-622N can be used in either transmit or receive mode. The dedicated transmitter unit simply makes it easier to see and adjust all your settings, because an LCD screen tends to be a lot more informative than a handful of blinking LEDs.
Yongnuo has also built in a YN-622 receiver into its YN-685 flashes. Just be sure to get the Nikon version.
There are no other standalone receivers that work with a YN-622N-TX. Radio triggering systems are typically incompatible with triggers of another brand. And Yongnuo's TTL, manual, and RT radio triggers are all mostly incompatible with each other.
However, there are other triggers that do iTTL/HSS. A better solution, if you have to stay in the same price range as Yongnuo and can't go with Phottix, PocketWizard, or RadioPopper triggers which do have compatibility with their manual trigger counterparts, is to go with the new Godox X1 TTL/HSS triggering system. Godox is building them into all of their new lights, both manual (with HSS and remote power control) and HSS/TTL. And has backwards compatibility with their XTR-16 manual receivers. So you have a choice all the way from the super-cheap TT600 (a YN-560 equivalent manual speedlight with built-in power control and an X1 receiver), the TTL/HSS TT-685, the battery-packed V850II/860II models, the AD360 bare bulb flashes, up to the AD600 TTL/HSS battery-powered studio strobe or QT-600 monolights. And system-wise, your choice of Canon, Nikon, or Sony (multi-interface hotshoe), with plans to expand to the Pentax, Fuji X, and Four-Thirds flash systems. That's a lot more expansion capability than Yongnuo offers.
Understand, too, that in order to have iTTL and HSS with TTL-capable triggers, your flash and camera body also have to be capable of iTTL and HSS. If you're shooting a D3x00 or D5x00 body, you can't do FP/HSS at all because the camera body doesn't support it. And a single-pin manual-only flash can't do either iTTL or HSS.
It should be noted that a Nikon-compatible YN-685 speedlite is now also available on the market.
It is compatible with the RF-603N / YN-622N systems (including YN-622N-TX of course).