I just did a whole bunch of Googling for the best price today. Jn4jenny found it. But if you want the individual vials, look at Medi-Vet. They sell individually at roughly $42.85 per dose. FarmVet has 7 vials or more at $41.50 per dose. Both have free shipping over $100, but for FarmVet the entire order has to be under 4 lbs.
I have to purchase I.M. Adequan on a regular basis for my little mare. I like buying from Allivet. Adequan Equine 500mg/5ml 7 x 5ml Vials is $289. There is free ground shipping for orders of $99 or more & sometimes coupons. FarmVet also has good prices if you buy it in bulk (28+ vials at a time are $38.75 each).