I have used this with great success http://www.ebay.co.uk/itm/Acid-Tone-stain-1L-Ebony-/250482892063?pt=LH_DefaultDomain_3&hash=item3a51f19d1f It comes in a variety of colours.
I don't know about restorers but the green and blue slabs always fade after a few months. That's why they are not usually sold . The red, brown and yellow ones also fade but keep much of the colour for years.
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