Offhand I don't know of an online store that'll have them; but, I know what
my university does to get coveralls (which are the unofficial uniform of frosh week and homecoming) is arrange fittings at a local used clothing store
Phase 2. They're able to take care of the silk screening of the year motto, etc. and can cope with pretty low numbers, too (there was < 50 Computing frosh and they were able to get a different motto than other frosh) with a short turn around time. You've probably got a similar store locally.