deHavilland Mercury Type 85 tubes - Help needed

One of the two Type 85 output tubes of my deHavilland Mercury preamp has died and am unable to get a matched pair of these tubes or a pair from the same manufacturer. Tube depot, Tubeworld etc.. don't these in pairs or matched.

All i can find is used single units on ebay.

I would very much appreciate your help and advice and also whether there are any substitutes i can use and whether its OK to mix different brands.

many thanks