I am the lead guitarist in a hard rock metal band.

I use a Boss MD-2 distortion and the thing is, when I need to solo I never have enough volume to be heard!

What should I do?

As in what equipment to buy
I'm hardly going to go over to my amp and turn up the volume when I need to solo!

Isn't there a volume boost pedal?
Turn your guitar down, and turn your amp up.

Then when its time to solo just crank the volume on your guitar.
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If you cut your mids you'll have this problem. Most of the time you can just eq your current set up to boost the mids and highs a little more. That will help you cut through the rest of the band. Adding a volume pedal or a boost pedal will just be extra crap you need to screw with.
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- Albert Einstein -
That's what I do, I turn the volume down on my guitar for rhythm parts so that I blend in to the background, plus it gives more of a percussive, less gainy sound and then crank it up for lead parts, this also means I don't have to stop in one spot at a pedal board all the time.