USB to SPDIF adapters abound now, in many configurations, and prices. Obviously you "usually" get better performance with higher prices. There are several variables which determine the "quality" of the transformation of the signal from one digital format to another. Separation of the computer bus power supply from the outgoing signal is usually receommended. (galvanic and/or transformer isolation) There are several output styles - SPDIF (Coax[RCA or BNC], Toslink, AES/EBU) and I2S. Picking one for your specific DAC may rule in or out a specific device.
Things to look at:
1) power supply (battery/wallwart/USB bus/other custom external PSU)
2) clock chip quality/upgradability
3) Which USB receiver chip or system.
4) Async vs synchronous. Async usually requires a driver for Windows, but usually not for MacIntosh
5) If the adapter "reclocks" the digital stream (what happens in async devices) then jitter is usually decreased.
On top of all of that, depending on your system and source, you may or may not be able to tell a difference using a "better" interface device. There are many passionate arguments by the different factions, with some common ground, and some wide differences about issues that I have a hard time even understanding.
There are several well regarded converters in the $300-600 range which will work well for most systems. If you have to have the "best" for a very resolving system, I would look at the Empirical Audio Offramp 5 or the Berkeley Audio Alpha USB devices.
I currently use a Halide Bridge with my NOS DAC, but have an April Music Stello U3 on the way to use with the AES/EBU input on my DAC, which is reportedly better. When my listening room improves, I may upgrade to one of the 2 converters above.