Jens is correct, it's hard to get a propane to really go boom. The tanks are designed to vent (which makes a big flame, but not an explosion). Even if you release a whole bunch at once, propane has a narrow combustion range, so lighting it off is harder than you might think. Getting a BOOM rather than a FOOMP is harder, but a FOOMP in a confined space can be pretty damaging.

That said, I'd bet if you dedicated yourself to making propane tanks go BOOM, you could eventually get a reliable system that would demolish a garage with a 5 gallon can. Perfecting that before someone noticed your efforts would be a different story.

Other options:
Hydrogen has a MUCH broader combustion range, and can easily be produced, either by electrolysis (slow and power intensive) or from commonly available household materials. Storing and transporting the shear quantity equivalent to a propane can would be present difficulties, but you may not need to. You can also get it in bottles, but not in hardware that stores I know of.

Acetylene (which isn't supermarket, but can be found in hardware stores here) is also easier to make go boom. Welding tanks have various safety features, but producing a garage leveling boom wouldn't be much trouble. Unlike propane, it's just itching to take your face off.

An alternate approach for things like propane is to add oxidizer, i.e. a bottle of pure O2 (again not supermarket, but easy enough to get.) If you bust open an 02 bottle and a propane can in close proximity, the whole "mix your propane with air" problem pretty much goes away... along with your garage. Of course, if you are buying an oxygen bottle, you might as well go with acetylene.

Highly compressed oxygen (or even air) in the presence of various common substances results in an explosion hazard.

Non combustion "explosions" of things like boilers or air tanks can produce garage leveling events if their safety devices are disabled.

Overall, I'd say you can make plenty of stuff that could get the job done. Most of it won't come close to TNT pound for pound, but it doesn't need to.

Anyways, most of this discussion is silly. I mean for god sakes you can buy both black powder and gun powder by the kilo. Any decent sporting/gun store will sell this to you, and black powder can be made with relative ease using little more than what one might find at a campsite (and a lot of time).

As impressive as a flour bomb might be... couple kilos of a properly packed and compressed powder will do wonders on your garage.

Jimmy the Creepy Kid Living Down The Street, well, that's another story altogether. About the only thing he's going to do is blow himself up.

[That was an ANFO bomb, and the report that he used a sparkler as a fuse seems to be, presumably deliberately, leaving out what the sparkler was actually stuck into. ANFO is a low-sensitivity explosive; you can't set it off with a mere fuse. There isn't a "blasting caps" aisle in Bunnings, either :-). -Dan]