The best for the money would be these -
Etymotic Research, Inc. - ER•20 High Fidelity Earplugs
I got them for £10 online somewhere (can't remember where though, I thought it was stringsdirect, but I can't seem to find them on the site) and give a pretty flat response while cutting down the volume
The step up from that would be the custom moulded ones, of which Etymotic do (Not sure who else does them, I have only really seen these ones) the main differences between these and the ER20's is that they are custom fitted to your ears, so are more comfortable to wear, and you get a few different levels of filters instead of a set 20dB cut (9, 15 or 25 dB of cut)
These will cost you, around £150 when I had a quick look a while ago, though you might find them cheaper, those need a mould taken of your ears (which can be done in places that fit hearing aids and the like normally) which gets sent off and made into the earplugs
David