Here is a quick and easy way to recycle caulk tubes for re-use. Apply whatever substance you like in a caulk like manner even if you can't buy it in a caulk tube. Re-fill as many times as you want.
I wanted to be able to apply drywall compound from a caulk tube to do some touch ups on one of my projects. Nobody seems to make drywall compound or spackle in a caulk tube. Empty caulk tubes are also not readily available. None of my local hardware stores carry them. I found a few suppliers selling them online, but they were pretty expensive with shipping factored in.
The only supplies you will need are:
A caulk tube (preferably with water based contents)
An air compressor with an air nozzle
Optional: bottle washer attachment for faucet
Step 1: Empty the caulk tube
Empty the caulk tube of as much of its contents as possible.
If you just want an empty tube to use, but don't have a spent one from a project, you can buy a new tube of painters caulk for $1.38 at Home Depot. Obviously, it is least wasteful to use the caulk for something rather than just throwing it out. But, if you don't need the caulking and need the caulk tube, this is the cheapest way to go. Buying empty tubes online and having them shipped will run you at least $3/tube.