I have the same problem. If you find a good source for quality drawstring pants, without an accompanying jacket, please post it.
Actually my sensei's got the same problem as well (I don't - as I am still a beginner and my pants are not worn out yet). You CAN buy pants alone from several on-line martial arts suppliers, but they usually charge upwards of half the price of a jacket+trousers combo which is kind of expensive.
Lately I have been toying with the idear of sewing some myself. After all white cotton fabric isn't that expensive and it should be possible to figure out the pattern.
However if anybody out there should have a pattern for gi-trousers I would really like to get a copy. Otherwise I guess I just have to cut up an old pair and use it to figure it out.