Grille cloth is harder to find than it used to be, when dozens of radio patterns were available. AES is one source, although their stuff is mostly for guitar amps:
http://www.tubesandmore.com/
Radio Daze has reintroduced some radio patterns:
http://www.radiodaze.com/default.aspx
eBay is another source. When the old suppliers announced they were going out of business, I suspect some people bought up stocks and hoarded them for resale.
Phil Nelson
Phil's Old Radios
http://antiqueradio.org/index.html