A quick google search shows that 670nm lights are quite commonly made and easy to obtain, amazon has a plethora of them (although 660nm is considerably more common).
I emailed the professor who did the research and asked about 660 vs 670:
"Yes I am aware that the 670nm LEDs are hard to come by, they are also rather variable between batches which has caused a lot of problems for us as we now have to test extensively. I think 660nm will work. Question is, will it work as well as the 670nm? I suspect not. But also I do not know if this can be compensated for by longer exposures etc etc… There are a lot of variables here that we do not know about"
