Reddit mentions of 20 pcs Photo Light Sensitive Resistor Photoresistor Optoresistor 5mm GL5516 5516

You could actually make these pretty easily. You'd just need a blue LED, a photoresistor, and a couple of volts. I'd say go with an LR44 since they're cheap, small, a couple volts, and will lie flat in the shoe. Do the math or play around with them to see how many you'll need (Depends on how you set up the circuit, though I'm guessing one in each shoe will be fine).

So the circuit is pretty simple with the three circuit elements (LED, photoresistor, battery) in series. It's a little tricky though, since the photoresistor's resistance increases as the amount of light hitting it increases. So if you just wired everything up in series, the LED would turn on when it's light outside... that's the opposite of what we want. But you can make a voltage divider circuit and include the photoresistor in that, so that the brightness of the room and the brightness of the LED are inversely proportional, which is good.

Once you have the circuit, you just have to find a way to somehow put it in the shoe. Take the sole out, drill a hole in the tip for the LED, and drill a hole on the side for the photoresistor. Then lay your circuit flat inside the shoe, position the LED and photoresistor accordingly, and then glue them in so they stay in place. Cover the circuit with a strip of duct tape so it stays down, and then cover that with a Dr. Scholl's to maximize comfort and durability.

If anyone actually wants to do this, let me know. I've linked to the parts you would need, and they're all pretty cheap (the Dr. Scholl's are the most expensive part). I can draw up a circuit schematic if anyone's interested so then it would just be a matter of you soldering, drilling, and gluing it together.

Edit: Might also be a good idea to include a switch so you can save on battery life. There are all kinds of switches out there to choose from, and you can put it pretty much anywhere in the circuit where all the current flows through (so avoid putting it in the voltage divider section). So that part of the design is totally up to you. Maybe drill another hole in the side of the shoes for the switches, maybe have them be toe-activated, it's your call.