Are there remote codes for the Roku box?
I would doubt it. My experience is that if your device is newer than when the remote was designed (much less made or purchased), your chances of finding a correct code are pretty marginal. And since the NetFlix Player hasn't been around that long, it's a little doubtful that the remote makers already have "codes" for it...
However, sometimes the new devices reuse "codes" from other (older) devices. While I have no idea if ROKU did this with their "Player", you could always search for a proper "code" if your remote has a "code search" feature (where you keep hitting keys until your device responds, and then read out the "code" from the remote). I'm not convinced that would work, but IMHO you have nothing to lose (except your time) by trying.
Of course, if you get some higher end "universal remote" that has the "learning" feature (where you not only can program via "codes", but also by "training" the remote to mimic signals from other remotes), you certainly can "train" such a remote to mimic the NetFlix Player's remote. In fact, that's what I did at my house, as we have a couple remotes (I think they were about $20/each, i.e. neither the cheapest nor the most expensive universal remote models) that not only will let you program via "codes", as well as the "code search" feature, but also (if you so desire) let you tell the remote to exactly mimic another remote. This is a little tedious to setup, as you have to "train" each button press one at a time, but if/when your remote has a "learning" feature it can be "trained" (one button at a time) to mimic the signals from the NetFlix Player's remote. And once such a remote is "trained", you can then use that universal remote instead of the one that comes with the NetFlix Player (and also use that universal remote for any other devices you like).