How can I get the materials required for crafting TNT easily?

As long as you near a river or desert sand should be easy to get. Just make sure you use a shovel. Gunpowder is the harder part, but fortunately there are several ways to get it:

• If you manage to find a desert temple looting it can sometimes give you all the TNT and gunpowder you need! Just make sure you dig below the treasure room and DON'T step on the pressure plate.
• Creepers drop from 0-2 gunpowder. Not a lot but if you make a mob farm you should get it at an adequate speed.
• If you don't know how to make a mob farm then you can spend a few nights up killing creepers. If you have and iron sword or greater you can rush the creeper (rapidly attacking them). If you have wooden or stone sword put in a few hits then back away then attack again.

These are the best ways to get gunpowder and TNT. Hope this helped!