Not all technology is available for research and production. This is where the Black Market comes into play. Captains are able to buy whatever is on sale. Some of the offers are better than others, and they change frequently.
Only items with a purple border are special units.
When a special unit is purchased from the Black Market, you don't just get the unit, you also get a license for that unit. You can build one of each unit for each license you have. Each purchase gives you an additional license for that unit, so if three are purchased and later destroyed, all three can be replaced. This will cost gold and iron, but it won't be necessary to spend more Gems to replace special units. See Replacing Lost and Destroyed Units.
Purchasing from the Black Market
Special Units are also known as "Black Market Units" due to the fact that they are all mainly available from the Black Market. What separates these items from the rest of what is available on the Black Market is the fact that these special units can never be researched and built normally. Many items on the Black Market (all of the ones without purple borders) can actually be unlocked for construction once the Captain reaches the correct level and does the necessary research.
Making a purchase is easy, all it takes is Gems and a click of the "Buy" button associated with the item.
Replacing Lost and Destroyed Units
Special Units purchased from the Black Market that have been lost in battle can be rebuilt without needing to be bought again from the Black Market. Vehicles are available in the Engineering room and Infantry are available in the Training room under a "Special Units" tab like the one highlighted in the below picture.
It will cost some resources, and will proceed like other construction or training; click on the item to add to the queue, once processed through the queue the item will be placed in your Storage.
Other Ways of Getting Special Units
You can also get special units from Booster Packs, or by winning difficult raid battles from Jasmine's Gentlemen's Club. Special Units may also be gained as rewards for completing Cooperative Boss Battles.