Martin Tarenskeen has done great work in providing a large library of sounds for the Reface DX.
First, there is his Reface DX Legacy Project, which recreates the sounds of many Yamaha 4-op FM synths (including the DX100, TX81Z, FB-01, and more) in the Reface DX format. There are hundreds of sounds available, for free, but I also encourage making a donation on his site to support the project.
Second, Martin has also done the work to convert over 6000 sounds from Yamaha's SoundMondo website into sysex files that can be loaded onto the Reface DX from Patch Base, or Martin's Ctrlr panel, or any sysex dump utility.
Download the firmware updater software for Windows or Mac as well as the manuals directly from Yamaha's website here.