Teenage Modeler
I have no idea as of where to buy resin for dioramas.
Best bet is a store like Hobby Lobby, or Jo Ann, or a similar arts & crafts type store.
Resin is not cheap.
Resin can be very fickle, too. It needs careful measurements, you really want a stack of disposable containers for mixing, which have to e made of things that do not react to the chemicals in the two components in resin.
Resin generates heat as it cures (many resins also generate stink while curing, which can get the project evicted to outdoors). Anything that lands on the curing resin is there for the duration, so you need to be able to drape it to keep dust, debris, pet hair, random boojums, etc. from landing on it.
Many acrylic (water-clean up) paints will color resin. Less Is More. Adding enough color to "see" it wil lgive a nearly opaque end product.
As an option, there's an Art product, Acrylic Gel Medium that can be used as a stand in. If not so handy bey being able to be poured.