Certain TotalBoat epoxy systems, such as TotalBoat ThickSet, TotalBoat ThickSet Fathom, TotalBoat MakerPoxy, and TotalBoat TableTop Epoxy are available only in kits - the resin and hardener components are not sold separately.
Kits are a convenient way to get the epoxy resin and hardener components in the quantities needed, because they're available in different kit sizes. Some kits are even packaged with precalibrated dispensing pumps and mixing tools. If the metered pumps are not provided in the particular kit you're using, 2:1 pumps, 5:1 pumps, and 3:1 pumps can be purchased separately, as can the plastic mixing pots and stir sticks.
For the TotalBoat High Performance 2:1 Epoxy system and the TotalBoat Traditional 5:1 Epoxy system, the resin and hardener components are available separately and in kits. Because these components are packaged according to group sizes, check to make sure the group size letter (A, B, C, for example) on the epoxy resin matches the group size letter on the hardener for the system you're using.