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 by being at low rank. But that is an acceptable compromise taking into account the vast number of other things you can buy.