Utrecht is famous for their canvas. Generally they have a huge sale at the beginning of traditional university semesters (Fall and Spring.) Right now Utrecht is having sales, but you might also try Dick Blick, Perl, or The Art Supply Warehouse. Those are just a few major companies I know and trust.
If you’re painting anything beyond (and including) 3.5’ x 3.5’, then stretching your own canvas really becomes more economical (after the initial tool investment.) Smaller than that, you should definitely purchase them pre-made.
For a measure of material costs, just yesterday I went to purchase a pre-assembled/primed canvas at 4’x5’, but it would have run me $75 minimum, and $90 for decent quality (@sdeutsch, this was at A&C supply mind you and included the 70% discount, so it really must vary by region.) Instead, I purchased some raw canvas and stretcher material for about $50.
At large scale, however, it’s really important that the stretcher is well constructed and braced with corners and/or cross bars.
I’ve yet to find a decent tutorial/reference for you regarding larger scale stretcher construction, so I might have to post/make my own. I’ll keep looking, and get back to you if I find a decent article or get around to posting my own.