In case you haven’t noticed recently, PS4 pre-orders (for bundles and the standalone console guaranteed for launch day) appear to have been halted at both GameStop and now Amazon. The “Standard Edition” pre-order, which doesn’t guarantee any sort of launch window unit at all, is still available.
After what appeared to be a huge, sustained burst of pre-orders once the console’s details and pricing were revealed during the E3, the allocation has apparently dried up (for now).
Your best bet is to sign up for an email alert or check out other retailers, but beyond standing in a line (probably in rain and/or snow) on launch day, it’s entirely possible that it will be difficult to get your hands on a reasonably priced PS4 without a pre-order… or catching hypothermia.