[...] I'm thinking of just using a computer power supply and regulating the 12V line down to 7.5V and calling it a day. Problem is...I don't know how to do that.
a) Have links to tutorials I can use to build it?
b) Have a better option that doesn't use batteries?
The few modify-a-PC-PSU-tutorials I've seen have been highly specific towards a particular make and model and if what you've got is different, it won't help you any.
Start by opening the supply - after it has been off for at least an hour and remember to discharge the electrolytics, not by a screwdriver, but with a suitable resistor (you want them to still be in good shape).
Find the switch mode regulator chip. It will have traces to the switching transistors and via a resistor network of some sorts to the output.
Find the datasheet for this chip. Look up at what pin the feedback voltage enters the chip and trace out what's connected to that pin. Usually it will be a potential divider that divides the output down to whatever reference voltage the chip use for comparison.
Calculate the upper transistor in that divider for 7.5V, swap it and Bob's your uncle.
Don't change anything until you have the datasheet and finally, remember a fuse on anything you connect
, there's a lot of current in such a beast.