Recently, I had the necessity to power Fuelino with 12V voltage, but having only a PC with me. The purpose was to make some tests "on the desk": since recenctly I travel a lot, I don't have space to bring, with me, a complete "220V AC to 12V DC" power supply. So, I searched on Amazon Japan and found what I was looking for.
The product name is: "Haobase 2A DC-DC Boost ステップアップ 転換モジュール Micro USB 2V-24V To 5V-28V 9V 12V 24V". It consists on a small module capable of converting an input low voltage (for example: 5V, from micro USB connector), into a higher output voltage (for example: 12V, 24V). The voltage regulation can be performed using a potentiometer. The price is only 650 Yen (about 6 Euro), with Amazon Prime.
If the load, on 12V side, just absorbs some hundreds of milli-Amperes, of electric current, it is possible to receive the power directly from a PC. However, it is important to make sure that the power drawn by the load is not too high, otherwise the USB port of the PC will be overloaded, and might be damaged. In my case, I estimated the complete electrical current, on 12V side, to be around 200mA (this is the consumption of Fuelino, plus the LED connected as a load instead of the fuel injector). This corresponds to an electrical power of 2.4W. This power (plus additional power due to the non-perfect efficiency of the DCDC converter) is drawn, on 5V side, by the PC USB connector. As a simple calculation, taking into account an efficiency of 80%, I estimated the power absorption on 5V side, which becomes 3W, corresponding to 0.6A electrical current drawn from the USB port of the PC. It should be acceptable.
The following picture shows how the DCDC converter is connected to Fuelino Proto2. On the left side (5V), it is connected to the PC using a Micro USB cable. On the right side (12V), it powers the Fuelino board.