Try multiple shops. Try multiple brands and models.
Head shapes vary, so it's possible that one "66cm" (26in) model will be too small and another "66cm" model will fit. It's worth trying multiple shops since most shops will only stock one or two brands of helmet and only carry one model intended for larger heads. Having an idea whether your head is long or wide might help. It's hard to measure your head properly with a measuring tape, however.
The Bicycle Helmet Safety Institute website has a page about helmets for large heads that gets updated when new models come out. It seems that 66cm is the largest size for bicycle helmets.
If you can find any sort of multiple options, a helmet that fits lower on your head and wraps more around the sides and back is better than one that perches on the top of your head.
If you can find a motorcycle helmet that fits, that should provide as much or more protection as a bicycle helmet. I believe motorcycle helmets are measured against a higher standard. The problem with motorcycle helmets is that they're heavy and tend not to have as much ventilation as a bicycle helmet, so it might be uncomfortable.
Note on sizes: 66cm = 26inches = XXXL = 8 1/8 UK = 8 1/4 US = 9 1/2 FR. Those last few are hat sizes, but many helmets seem to have the hat size type sizing info easier to find. Look for helmets with those sizes or larger.
I have big head issues, but luckily most of the big-head helmets do fit. I've had good luck with the "Specialized Max" line even though the official sizing printed on it is smaller than my hat size. I think they're a more rounded rather than circular shape on the inside and that makes all the difference. I'm not recommending this model for you, however, I'm recommending you check out the big helmet options at multiple shops.