Changing armor whenever you like is great, but what if you want other items to be available at any time in the game? Well, here's a solution in the form of the AIIS mod that literally puts all the items into the game's shop and readily available for purchase.
Of course, there are a few restrictions. For example, you can't buy high-rank items while at low rank. But that is an acceptable compromise taking into account the vast number of other things you can buy because of this mod.