If the only problem is worn out knees, this might help:
At my first dojo we did a lot of shikko and surwari-waza; you knew you were finally a regular student when your knees wore out of your gis, and of course the gi sensei prefered us to wear came only as a set. But you could take the pants to the very nice tailor who sewed our dojo patch on the jacket and he'd sew patches on the knees. I was quite proud when i got to the second set of patches on one pair of pants
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One student with bad knees gave his gi pants to his grandmother, who not only patched them, but put in thick padding when she did so.