They are not 3.7 volts. Rechargeable lithiums are 4.1 or 4.2 volts when fully charged. The voltage falls as charge is expended. Typically you should stop taking current from them when the voltage reaches the region of 2.6 to 3.0 volts.
I get mine from Monoprice (they're located near Los Angeles). For "wearable" applications, I usually use a 4Ah unit. For a Pi2B plus 7" RPF display, I use a 10Ah or 10.4Ah. Pretty much any commercial unit will do, so long as it will deliver its specified current while maintaining 5v output. You do have to be alert to the max current rating. Smaller units (like the 4Ah ones I use) tend to be limited to 1A, while larger ones can supply 2A or more. Be sure to read the actual specs. After that, it's just a matter of how long your particular system will run on a given pack.
"Total combined output - 3.4A"
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