You can get 2.5% Benzoyl Peroxide at places like Boots. Oxy On the Spot is the one I use.
You can do but it's probably about 10x more expensive as the stuff from there is about £8 for nearly 250ml where as i bet you are paying something like £4 for about 10 ml (at a guess).
I don't have acne but do get the odd spot and it works a treat, it gets rid of them in a day or so much quicker than letting mother nature do it's work.
	
 
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.  I dunno how much volume that is though... probably very low.  Having said that, a 20g tube does last ages.