On the Internet (whoever has the best deal) with a credit card (which I pay off every month). :-)
The way I see it, unless you absolutely NEED a piece of equipment you probably shouldn't finance. If photography is mostly your hobby, then paying an interest rate on a hobby isn't a great decision.
I don't know whether one can really say that ... people do a lot of weird stuff over their hobbies / stuff they don't really need ... like car(s) purchased on finance, for instance. Its just a personal preference.
I usually wait for a deal and/or pay out of my savings account.
Hence prefacing that sentence with, "The way I see it." It's certainly a matter of personal preference for me, but photography is a hobby. While I don't claim to be great with money, I try to finance as few things as I can (unless I can get some ridiculous interest rate). Big ticket items for most people need to be financed: Houses, cars, usually anything over a certain threshold. And while that lovely 600mm f/4 IS II would fall under the needing financing category for me, I simply can't justify it as I don't need it to sleep in, drive to work in, or make money with.
But, this is entirely a personal decision. If someone is comfortable putting a camera, lens, or other equipment on their credit card and paying it off monthly, then go for it. But that can quickly become a slippery slope... especially with photography, as I've found it can be quite addictive. :-)