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
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.