You can either use a zener diode or a voltage regulator.
For the first solution you need a 3.3V zener diode (like the 1N4728A) and a resistor to create a voltage regulating circuit (http://www.electronics-tutorials.ws/diode/diode_7.html). With the second you need a couple of capacitors and a voltage regulator (like the LM1117-N-3.3).
The same can be achieved with a level switch board, there are cheap ones available on eBay.
If you are in shortage of those you can leverage the intrinsic diode voltage drop and use just a couple of diodes between the 5V line and the 3.3V line of the nRF24. Normally a diode cause a voltage drop of about 0.7V, two in series will produce a 1.4V drop: 5V -1.4V = 3.6V, close enough to your target 3.3V.
Among the above, the best solution is using the voltage regulator. The other two either favor voltage precision (zener diode) or energy consumption (diodes in series).
Please consider I'm not an expert in electronics: I'm just sharing what I've learned so far.
I recently purchased two nRF24L01+ transceivers, but it turns out they must run off 3.3V.
Unfortunately, I planned on using it with an Arduino Pro Mini which has a higher voltage (5V).
I’m using a 9V battery to power the Arduino Pro Mini, but is there any way I could make it work with the 3.3V transceiver module? I prefer staying away from AA(A) batteries.
Voltage regulators (eg.the LM317) cost about $0.30…