As Vertex is now owned by Motorola
, you'll need to jump through the hoops of getting a mol account. I'm not aware Motorola now charges for Vertex software (it use to be free)? You might approach a local dealer, explain you're a HAM and it's going to only be used for that purpose (hobby, disaster, noncommercial). A lot of times dealers will help out HAMs with reduced costs or even free when it was just a simple download for them. A little word of mouth for them if they help you out is always good. There's also the possibility to buy used gear.
There is an online subscription site, but I have a real problem with people who sell stuff for $$$ that they got for free. There are a bunch of ebay sellers that also do this crap.