How to achieve a realistic wood grain effect is one of the most common questions asked when discussing early aviation builds.
Decals are one option, and Spada Decals have released a series of decal sheets that address this.

The Wood Effect decal series from Spada Decals comprises of seven different
styles of wood grain, and includes interior and exterior types.
Designed to be applied over a painted base coat the opaque decals are laser printed
on clear background decal sheets, allowing the grain to shade to the background colour.
The seven different effect patterns are available in any scale, with each sheet measuring 8 1/2" X 5 3/4". The actual decal area being 8" X 4 7/8" (205mm x 125mm)

The sheets are available direct from Spada's website on a pre-paid basis.
Prices quoted being $12.25 USD or 9 Euro plus shipping costs, and order processing is normally approximately two weeks.

Images can be found on Spadas website but because of copyright restrictions cannot be reproduced here.