Many of the most popular resin systems we sell (5:1 mix ratio and 2:1 mix ratio, for example) make the resin and hardener components available separately and in kits.
Typically, the resin and hardener components are sized in groups (Group Size A, Group Size B, for example) so that, when the resin and hardener components are dispensed at the proper ratio (either by volume or using the calibrated pumps), both bottles will be emptied at the same time, and there's no wasted product.
If you're new to working with epoxy resin, kits are a convenient way to get just the right amounts of resin and hardener for your particular project, and some are packaged with handy measuring pumps and mixing tools. Be sure to check the details on your particular kit to see exactly what extras the kit contains. For example, are the metered pumps included? If not, you can purchase them separately.
If you already have mixing cups, mixing sticks, gloves, and pumps, and all you need is more resin and hardener, you can purchase these components separately. Again, because these two components are packaged according to group sizes, check to be sure that the group size letter (A, B, C, for example) on the epoxy resin you're using matches the group size letter on the hardener.