According to bulbapedia, Mom will spend money every 10,000 pokebucks (or whatever they're called). Some items she will only buy once, while others can be bought an unlimited amount of times. To get any one item, it's random, so just keep beating the E4 until you get it, I guess. So, in short, to get any item, just collect 10,000 more poke-currency and go to the nearest pokemart.
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