You could always track down some suitable material and make your own. (leather, vinyl, canvas, etc...)
Or have an upholstery shop make it to your specs. (bring an old strap along so they can match the peg hole pattern)
There's someone else, but I'm damned if I can remember their name - I'll measure up my one that I got the other week in Denmark Street & get back to you.
I've made plenty of straps from cotton webbing and leather scraps.
My current favorite, though, is a strap I had custom made to "resemble" Peter Green's- IIRC it's 57" and cost me $35. I don't think it would have cost more for the extra 3" you're looking for. Unfortunately, I don't remember who made it for me- some dude on the intArweb.
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*Recipient of the 2006 Time Magazine "Man Of The Year" Award*
Dreadful website, but does have contact info. I measured one of my XL straps & it comes in at a tad under 2.75" wide by a maximum of 59". I picked up two & got change out of £30.
They seem to be fetching up in retailers all over London, so you might find 'em in Bristle - Try Treblerock Music up near the Highbury Vaults pub in St. Michael's Hill.
I'm after a 60" long strap 
