Go into household management and check each and every homeless townie. Chances are you already have at least one townie alien if you have GTW installed. That is how I found mine, completely by accident.
If that is too tedious wait until its alien night at the bars and participate. You should meet at least one or two there. The game should generate them if they don't already exist.
You also have a chance of meeting someone if you go for the doctor career also the one that takes you to alien world.
Other than that, you might want to try the testingcheat and whisk someone into CAS and try it out. Might be able to clone the townie you want as an alien and then change that one into alien. How, I have no idea, because I really haven't tried it. But it is a cool concept.
