Spectrum Transmitters work with DSMX protocol receivers.
There three main sources for these...
1. Spektrum brand
DSMX receiver
2. Orange brand
true DSMX receiver
3. Lemon brand
true DSMX receiver
Spektrum brand is obviously the surest way but the most expensive. Different people have different opinions about Orange and Lemon but general reports seem to indicate they work quite well. Orange make receivers for other protocols used by other brands (eg Futaba) so make sure to only buy a true DSMX receiver.
Also a Spektrum brand TX will work with receivers that use the older Spektrum DSM2 protocol. Orange and Lemon still make these but you probably wouldn't buy one out of choice unless it is for specialized use (eg micro indoor or something) However if you buy used stuff there's a lot of DSM2 out there and you can use it if you want to.
There are possibly some off brands that also make Spektrum compatible receivers but probably best to avoid those unless you have good reason.
Receivers from other name brands like Graupner, FrSky, FlySky, Futaba, Walkera, HiTech, JR etc. will not work. They use different protocols.