So there is a small industry of men who go into The Wyld in search of things they can drag out and sell. Like say, trees made of pure gold, and whathaveyou. Of course, being exposed to the mutable nature of The Wyld results in mutations. (And then there's the problem of the wagon you cart into the place trying to eat you, or any other such insanity).
This raises an interesting point - is it easier to obtain the stuff from its source, or to steal it? If it's too difficult to get from the source, anyone known to have the stuff will have to worry about thieves - especially non-magical ones. Having the substance could be more trouble than it's worth. And don't forget that something this volatile is likely to be regulated by any nearby authorities.
This also raises the question of what causes the substance to explode. Being in the line of effect of a magic spell? Near a magic spell? What about permanent magic items, such as weapons? Can magic be used to detect the presence of the substance? The answers will detemine how dangerous it is to use the stuff - which in turn, determines if the risk is worth the benefit in using it.
For example, let's look at the frost giant lair you mentioned. Having bombs scattered throughout their lair is incredibly dangerous, not just for invaders, but for the denizens. Imagine a small party infiltrating the lair, locating the bombs, placing a fire glyph near each one, then coming up with a method for detonating the glyphs simultaneously. THEN the invasion begins...
And just imagine if the infiltrating party reaches the main stash of the stuff - instant nuke in a frozen lair.
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Even if such a plan is not workable, how many people really want to live near bombs that can go off at any time? I use a lock on my front door - yes, land mines would be more effective at keeping out burglers, but the risks (and the collateral damage) are prohibitive.
It seems the key is that, to be most effective, invaders have to be unsure if you have the substance and, if you do, how much you have. If they know it's there, they can prepare countermeasures. If they aren't sure, they might waste time and effort preparing for something that never happens, reducing the resources they have available for other things. Or they may lower their guard, only to (literally) have things blow up in their faces.
In fact, if this stuff is as rare as you suggest, bluffing may be the best option.
Everyone says they have some around - and 99% are lying. But which 99%??
