I honestly think it isn't THAT hard to score a UA-5. Just a little patience and you'll get one.
The UA5 was a huge success and a very widely used pre-amp. I don't want to say its a thing of the past because it definitly is not, but since they don't make them anymore, many people who owned them are upgrading to higher end gear. Therefore, you should have too hard of a time finding one.
They pop up on eBay on a regular basis as well. Just keep an eye out. When you're ready to buy, jump on one.
Never owned one, but they are a great box.