Sorry I missed this post of yours somehow...
I can get some meds for my pigeons over the counter here, but I have also bought online from
http://www.jedds.com/-strse-template/store_home/Page.bok -- they have great choice for coccidia and canker, but not so much for worms (Ivermectin is OK, but don't touch the 'zoles'). Moxidectin Plus is the best wormer, but I didn't see it on their list.
Postal costs from the US can be high, so it's better to buy everything you need in one go and use combined shipping. They deliver very fast.
I have also bought online from a Portuguese subsidiary of this British firm:
http://www.birdcareco.com/ They're very good for supplements, especially Calcivet.
The best spray for external parasites I've ever used I can get here in a pet shop.
But you may be able to order it online, if you can't buy it there. It's just called 'Harmless Insecticide SPRAY', from the Columbine range, by Versele-Laga. It's specifically for pigeon parasites, and very gentle on everyone else's lungs!
I know I've seen other recommendations from members. I'll add them here as I find them.
PS. I haven't used Coxoid before, so I was interested in what you wrote. I've used Coxi Plus (sachets to dissolve in water, 5-day treatment) and found it ineffective, but Coximed
(Diclazuril) worked 100%, and it's so gentle you can give it to convalescent pigeons and during the breeding seson, and you can repeat the treatment a month later, with no ill effects, if you suspect reinfection. And reinfection with coccidiosis and canker is the order of the day during warm weather, courtesy of those pesky pigeon flies.
I hope you and your pigeons are doing well.