The problem with some of the components that Apple uses in their designs is that they are either custom designs or only available with huge MOQ. You are correct that the Digikey models will not work as they are 2.8mm square and they simply will not fit. Some coils on the backside of the board are also hard to source because they are only 0.6mm high and everything standard is 1mm. It doesn’t sound like much but when you consider the density and clearance available, it makes a huge difference.
As for L1503, I order them from China. However, be aware that you never really know what you are getting because they are not “officially” labeled. I once ordered a batch from Jestar, a reputable supplier and when I checked them with an LCR meter, I realized that they were 1uH and not 15uH.
If you are going to using a different rating, keep in mind which ratings do what. For a coil, you have the inductance, the current rating and the ESR (equivalent series resistance). Using a slightly different inductance may or may not cause issues. The ESR is important because it causes internal heating and can have an effect on the overall performance (resistors, capacitors and coils interact and cause all kinds of filtering actions). Finally the current rating is what can make a permanent repair into a ticking time-bomb :>).
There are plenty of people selling this coil both stateside and internationally, that’s the preferred option.