How do I unlock an Optimus T? Is there a "vanilla" firmware I can use?
Unfortunately it appears your only option is an unlock code, I've just done a search.  This XDA thread has a couple ways to get into the unlock screen so you can enter the code.

Your options for get the unlock code are to ask the carrier the phone is locked to -- they might give it to you if they're feeling nice -- or to buy it online.  There are no reputable sellers of unlock codes online, of course, but you could search around and find someone whose experience seems reliable and use the same service that they did.