Some months ago I changed the strings on my Warwick bass from light Fenders to heavy Earnie Balls (strings); boy was I pissed off. Not because I didn't like them, but because I had few options left. Either stick with them, or go in a travesty to downtown to browse through some stores, which doesn't sound like a big deal, but trust me, when you live in Mexico City like myself, that is a whole day gone.
"I know!", I naively thought, "I'll get my strings online!" Yeah?, tough break, tough guy. Most stores like Guitar Center or Musicians Friend do not ship their stuff over here. Feeling like raising my fist in fury and claiming "latin power," again I set off to look for a store that did, and I found this site: Strings By Mail.
Tough break, no Fenders. However they did ship over here, and my feelings of being discriminated against had not faded away, so they could've sold grape fruit for all I cared, I was buying. I'm not going to lie to you, and since they're not paying me anything to write this review, I don't need to. For bass players, their variety is not the best. It's far from the best. Actually, they have 3 brands: D'Addario, DR, and Thomastik.
However, for guitar players, this would be a nice store for you to check out. They have a great variety of guitar strings of several brands and gauges, divided into three categories: Classical, Acoustic, and Electric.
They also have a variety of "other instruments" strings, so if you have an acoustic bass laying around, or a vihuela mexicana (I shit you not), you can pick up some strings for them here as well.
"What about other stuff, such as accessories, or books, or DVD's?" you're asking yourself, or not, but I'll tell you anyway: yeah, they don't sell just strings. I've found quite a few other interesting chochkies ranging from your usual stuff, such as guitar picks and capos, to the stuff that one would guess belong to another store, such as nail care and humidifiers.
What's nice about Strings By Mail, I found out, is they're big enough to have a wide variety of stuff (manly for guitar, though *raises fist in fury: "bass powah!"*), but small enough to treat you personally and not through a PR, permanent-smile person and automated e-mails. (When I got my strings (by mail), there was a hand-written message on the envelope thanking me for buying there).