TA Specialites makes 49T rings, and others in 1-tooth increments. http://www.specialites-ta.com/produits/plateaux_gb.htm

But to heck with all of that screwing around - I'm happy with 52/39 and 12-25 (10v) until it wears out, then I'm going to change the small ring to 38T and the cogs to 12-23. I won't have the gap between 17-19 when in the 52T, and 38/23 will be a low enough granny.