The supply bar can reach its maximum number of units which is 100. In order to get supplies, that can be spend on research/manufacturing, you can choose one of the two options:
Similarly to other businesses, you can get supplies by completing missions. Their course and complexity vary greatly. Some missions are simple, they require you to lose police, intercept a vehicle or to reach a place in a certain amount of time. However, the game also has much more difficult missions during which you must, e.g. assault Humane Lab or steal a tank.
Most of the missions can be completed alone. When you finish them you get 20 supplies. However, it is definitely better to complete them in a bigger group - each of the players that participate in a mission gets a chance to obtain another 20 supply units. In addition to that a bigger number of players can be helpful in completing harder missions.
Here, the choice is very simple. Money can speed up many things - the same goes for getting supplies. For $15.000 you can immediately buy 20 supply units. Pay $75.000 and your bar will be completely full.
Supplies can be used for developing technologies or manufacturing. More information about those topics can be found in dedicated chapters of this guide. In the meantime, it is worth to check out how quickly supplies are used during manufacturing process:
Number of bunker's upgrades
Time of using supplies
1 unit each 60 seconds
Bunker partially upgraded
1 unit about each 72 seconds
1 unit each 84 seconds
In other words, a full amount of supplies in a bunker without upgrades will be used in about 1 hour and 40 minutes. However, a fully upgraded bunker can use all supplies in 2 hours and 20 minutes. As you can easily notice, upgrades are very useful because you don't have to resupply all the time and you can focus on other game mechanics.