It might have taken six years to get there, but Apple’s (AAPL) wildly popular iPhone lineup is now available from T-Mobile USA. The world’s top-selling smartphone and its two predecessors can now be purchased off-contract from the “UNcarrier,” which recently gave its business model a big overhaul and did away with traditional contracts and subsidies. Under T-Mobile’s new model, the iPhone 5 can be had for just $99 down and 24 monthly payments of $20 — or, for users defecting from another carrier, the latest iPhone is available for no money down by trading in an older iPhone model. Apple’s iPhone 4S and iPhone 4 are also both available at T-Mobile stores and online beginning Friday.