The FLIR E4 OEM charger is rated at a higher current than the everyday phone chargers. This is to enable faster charging at up to 2A rather than the more common phone rate of up to 800mA.
I bought some high current car chargers that were designed for a tablet as they tend to have higher rated chargers. I think mine were 2.5A.
Most devices that can fast charge at higher currents will charge at a slower rate if a phone charger is used, but there is the risk of overloading the charger and it getting very hot. Much depends upon the charger design. As you state, avoid cheap car converters as they can be risky. I personally would not wish to wreck my E4 internal charge management by using a charger that suddenly goes series short and applies >12V to the 5V input.
HOWEVER, the E4 is permitted to take a charge from a standard 500mA USB port without harm to the E4 or USB supply. Some charge management IC's can detect when they have reached the maximum current draw (voltage drop detection ?) and will work fine below that level.
I decided to play safe and use a correctly rated car charger. I recommend that you hunt down a reputable brand tablet charger that is rated the same of higher than the FLIR OEM unit. You are then safe and will get the E4 charged in the fastest possible time.
Aurora
The USB "charger" is missnamed, the USB port on your computer or the wall wart you plug into the wall socket is simply a power supply at 5V with a rating typically of 0.5A up to 2.1A.
Here is a really important bit, THE CHARGE REGULATION IS DONE WITHIN THE DEVICE PLUGGED INTO THE POWER SUPPLY. NOT IN THE POWER SUPPLY.
Read up about lithium battery charging requirements. Example of an implementation here:
https://www.adafruit.com/products/259 I'd be a rich man if I had 0.00001 penny everytime I see nonsense spouted about having to use the exact "usb charger" with xxx device.
No device should be designed relying on current regulation to be performed by the "usb charger" ( i.e. wall wart/USB port) .
I myself have become so fed up with all the USB wall warts for all my gadgets, we dont have enough space in the kitchen when we are trying to charge all our devices. I am making a 5V power supply brick with a bunch of USB sockets, I am fusing each port at 2.5A with a slow blow fuse, just incase a duff device goes short circuit.