Inventory sorting by item level can be achieved with this addon:
Crafting Material Level Display Updated
But the sorting method is not always working that well, esüecially with materials, as they got a level range from-to and the game will always resort the stuff if you add/move something in your inventory.
I'm using it in cobined with
AdvancedFilters (and here
a modified version where you are able to reset the filters quick by one click!) and some
AdvancedFilter plugins (check the "Optional files" or search addons with the name "AF" and get a list of possible plugins) to only show/select a special item level etc.