Skybet in the UK is the only one we've seen accepting wagers, posting online lines for the race.
Odds here:
http://www.skybet.com/athletics/boston-marathon
Women's Boston Marathon 2015 Winner
Mare Dibaba 1/1
Aberu Kebede 4/1
Buzunesh Deba 5/1
Shalane Flanagan 8/1
Joyce Chepkiru 11/1
Sharon Cherop 12/1
Shure Demise Ware 14/1
Caroline Kilel 18/1
Desiree Linden 20/1
Caroline Rotich 22/1
Aleksandra Duliuba 22/1
Men's Odds
Lelisa Desisa 6/4
Patrick Makau 11/2
Wilson Chebet 11/2
Yemane Tsegay 8/1
Bernard Kipyego 11/1
Gebre Gebremariam 12/1
Tadese Tola 14/1
Frankline Chepkwony 20/1
Zersenay Tadese 22/1
Meb Keflezighi 25/1
Abel Kirui 28/1
Dathan Ritzenheim 40/1
Vitaliy Shafar 40/1
Lusapho April 50/1
Dylan Wykes 66/1
Wesley Korir 66/1