I use petvetdirect.com for vitamins, pill pockets, flea and tick repellant and the like. I don't know if they carry prescription meds, but their prices are much, much better than my vets, they ship promptly and have very good customer service.
Prednisolone is a pretty inexpensive medication. I don't know what your veterinarian charges but I would think by the time you pay shipping you wouldn't be saving much. In addition all generics are not created equal so with all these online pharmacies that have popped up you may not be getting exactly what it says on the label.
Unless it was an expensive medicaton that my dog was going to be on long term I rather see my money go to my vet than some online pet supply company.
I like Vetcentric because your vet gets a cut of every purchase you make. This lessens the need for the vet to maintain a large inventory of drugs, yet still earns some profit. I prefer to support our vet if possible, and going through Vetcentric makes it a win win situation.