They have been cracking down on unlimited data customers over the last year or so. For about 3 months last year after the iPhone 6 was released, there was a bug on Apple's website that allowed anyone with an existing unlimited data plan or old nationwide style tiered family plan to add on a new line to that plan with unlimited data. People were using this bug to max out their family plans with new unlimited data lines (while also getting subsidized priced iPhones), and promptly selling or even giving away the unlimited data lines to other people on eBay, HowardForums, Reddit etc.
This was done via a process called Assumption of Liability (AOL) where you transfer billing and contract responsibility to another person for that line. Up until November 13th, 2014 the unlimited data plan would be retained during one of these transfers so someone who was not already a Verizon customer, or was a customer but didn't have an unlimited data line could obtain one easily. The people abusing the bug on Apple's website would rinse and repeat using this process while simultaneously selling their brand new iPhones they just bought for $199 for $600+ and because people wanted unlimited data lines, they could hand those lines under contract away for someone else to deal with (a lot of profit with this whole scheme).
Now however unless you already have an unlimited data plan, there is no way to obtain one. There are people on eBay who rent you a line on their family plan for an amount of money per month but you have to put your trust in them to not screw you over. I have read rumors of an unlimited data AOL working if it's being transferred to an account that already has a line with unlimited data but that doesn't help if you don't already have one and want one.
Verizon also does not allow you to directly use your unlimited data line's 2 year upgrade on your unlimited data line. There are ways around this where you can use make use of your "upgrade" and obtain an device at subsidized pricing and still keep your unlimited line. The easiest situation would be if you have a family plan with an unlimited data line, and a tiered data line. If the tiered data line has an upgrade available you can upgrade that line directly and then swap your unlimited data line sim card into the new phone. If the tiered line is already under contract, the Verizon iconic portal (easily found on google), allows you to transfer your upgrade from your unlimited line to your tiered data line, and proceed with ordering a phone. This will use the upgrade from your unlimited line and extend its contract by two years, but will NOT make unlimited data fall off your line. Once the phone arrives, you simply swap your sim card from your old phone on the unlimited line into the new phone and it works without issue.
The last method is called the "best buy method". There is a long thread on slickdeals about this, but the gist of it is this. You order a new phone from bestbuy.com using your unlimited line's upgrade directly on itself. The best buy website will warn you that you will lose unlimited if you continue, and will have you proceed to choose a tiered data plan ($30/2GB). When the phone arrive do NOT turn it on. Take the sim card that comes inside the phone out and destroy it. Then all you need to do is put your old sim card into the new phone and you have a new phone with your unlimited data plan. This method works because third party companies do not have direct access to Verizon's system, and the activation of the sim cards they send out is what completes the process of telling Verizon that you have used your upgrade and chosen a new data plan. If the sim card is never used with that new device that plan change never happens.