Is 3,6-Dimethylcatechol commercially available? I've looked through all my catalogues and chemexper gave also no matches. Other name for this compound: 1,4-Dimethyl-2,3-dihydroxybenzene.

Dunno about commercial availability, but it can be prepared from 2-Hydroxy-3,6-Dimethylbenzaldehyde or 2,3-Dihydroxy-4-methylbenzaldehyde:

J. Chem. Soc. 1615-1618 (1953)
J. Chem. Soc. 2042-2044 (1954)

I'll look next week in the library if this is a practical route (precursors not prohibitively expensive, not too much steps and too low yield).