What kind of earplugs should you wear at a metal concert and where can I buy them?

The "finned" silicon rubber style are very effective.
The further you push them in, the more attenuation they give.

A common type come attached by a chord so you can leave them hanging around your neck if ou want to remove them temporarily, and they are washable.

Examples:
http://www.ebay.co.uk/itm/Industrial-Earplugs-3M-Blue-Tracers-Detectable-Reusable-Silicone-Ear-Plugs-32db-/111382772367?hash=item19eeee3a8f:g:xosAAOSwF1dUO~Sm

http://www.ebay.co.uk/itm/Reusable-Silicone-Ear-Plugs-Detachable-Cord-Case-Noise-Reduction-Grey-/261444396562?hash=item3cdf4ce612:m:m8qwTYE2hRchyHjTzB7bwGA

[Hard rock fan with tinnitus from working in industrial environments].

Have a look at the ear plugs offered at these places --

http://www.earplugstore.com/nasopl.html?_s_icmp=HOMEBAR1_MUSICIAN_PLUGS
https://www.alpinehearingprotection.com/music/
http://www.howardleight.com/ear-protection/earplugs
http://www.uline.com/Grp_227/Hearing-Protection?keywords=ear+plugs

You might want to buy some extras in the event one gets lost.

If you already have hearing damage the best thing is to see an audiologist (a hearing doctor) and have earplugs made for you. They make them like a hearing aid. They take a cast of your ear canals and custom mold them using the best technology. This is what all the professional musicians do to protect their ears. They cost more than the commercially available ones, but it's worth every penny if you already have a problem.

dB stands for decibel, a unit of measure for sound. It means that the earplugs only let in just so much sound and protect you from anything louder than the level they are rated for. The audiologist will give you the best advice on that too.

you need to stop going to concerts then - the next step is hearing aids and they don't even do much good for me in some situations - Tinnitus for almost 30 yrs