Reddit mentions of Reflexite Brand Conspicuity Tape: 2" x 12" White Diamond Plate

You'll find most if you look for "reflective green tape". Most types have a sort of diamond pattern on the tape. I'm guessing this would do: http://www.amazon.com/TapeCase-3277-Green-Reflective-Tape/dp/B007Y7HWMQ/ref=sr_1_2?ie=UTF8&qid=1453386782&sr=8-2&keywords=reflective+green+tape
This one will most likely work too (better at closerange, worse at distance): http://www.amazon.com/TapeCase-3437-Green-Reflective-Tape/dp/B007Y7IRSY/ref=sr_1_3?ie=UTF8&qid=1453386782&sr=8-3&keywords=reflective+green+tape
Don't get the tape that appears to be very lightgreen. Darker green is better. In my limited testing with tapes, without a diamond pattern works better for closerange, but this isn't conclusive and with some adjustment might even work well at large distances. I've improved a lot in the algorithms as well; if you like to test get without diamond pattern as secondary.
Alternatively, get a green light filter, put it over the flash and use plain white/silver reflective material (I've got similar materials in small localshops. The filter you get at officesupplies, the grey/white/silver reflective material from a $5 safety vest or just as tape):
Sample of safety vest; it's not tape but works: http://www.amazon.com/Neiko-Visibility-Yellow-Zipper-Reflective/dp/B0030AA3YK/ref=sr_1_3?s=hi&ie=UTF8&qid=1453387694&sr=1-3-spons&keywords=safety+vest&psc=1
http://www.amazon.com/Reflexite-Brand-Conspicuity-Tape-Diamond/dp/B00LGYS57Y/ref=sr_1_8?ie=UTF8&qid=1453387424&sr=8-8&keywords=reflective+tape
http://www.amazon.com/Transparent-Colored-Window-Film-Sample/dp/B00ORVX4RO/ref=sr_1_3?ie=UTF8&qid=1453387525&sr=8-3&keywords=green+transparent+film
Don't spend more than a few bucks on the material unless you verified that it works well. It's really all cheap stuff, but people tend to overprice and sell it bulk. A good way to test if the stuff works in store, is to load the apk on your phone and see if it tracks the marker in the store. If you're not shy of compiling, compile the apk with SHOW_CAMERA set to false in Config.java for a good debugging view of reflectivity.
Edit: Also, if you're testing in store, check for return angles. This means if you view at an angle of 45 degrees, see if that works well and so on.