The DIY product is called Creocote and is available in 5 Litre bottles Suziequeue. I have been told it is not as effective as original creosote, but it must have some preventative effect. Red mite love clean new wood as I have found out. Within the cracks in it they multiply rapidly, so anything you can do to make the wood 'dirty' has got to help. Make sure, if you can, to turn the coop panels upside down to get the stuff into the tongue and groove joints of the cladding. That may not be possible with some coops. I doubt Creocote will eradicate red mite once they are established though.
Surprised you can't buy proper creosote though Suziequeue -in 30L drums we got. A traditional agricultural merchants will have it and if you have a CHP number there is no argument.