I know of three place where you can order both(I believe two of them state that they can also supply the latest one, by Intervet)without an Rx--they are AgriMed, Heartland Veterinary Supply, and where I've gotten mine for the past two years, a vet supply here in my state.
You can google for both of the first; both have websites(the in-state one does not-I order from them because the shipping can be 'normal' UPS; it gets here the next day, with just standard charges--of course, I have to pay tax, since it's within my state! The OVERALL cost ends up pretty much the same from each place,(and believe me, I've calculated it...!) though I think the best actual product price was at AgriMed. It is higher for single doses VS 10 dose packs or vials(depending on what type you want)-between @ $14 to $16/dose for the stand-alone West Nile. There is also shipping costs; most require you to pay at least for 2nd day air charges, if you aren't located fairly closely nearby. they do routinely ship w/ ice packs; styrofoam boxes, etc. sometimes carry an additional charge.
Edited to add: If you just have a couple or so horses, it would likely be just as simple/cost-effiecient to get these from your vet. The more doses you need, the more it saves to buy them as described above.
(I just got mine from within the state; shipped Monday, it got here Tuesday, ice packs still nearly fully frozen! YES! )
Hope this is of some help....
Margo