Try Thousand Oaks Optical. They have different types of solar filters for many different sizes of scopes.
If you want to make your own filter, Baader Planetarium has instructions here:
Baader AstroSolar film is available from them or through many large full-line dealers. For instance, Astrozap brand cells that use this film mounted in cells for 8" SCTs as well as sheet AstroSolar film.
Used filters are often available in the classifieds of websites like Cloudynights.com and Astromart.com, as well as photography sections of sites like Ebay and Craigslist.
NOTE: Making and using solar filters should be done with every caution. Celestron assumes no responsibility for homemade solar filters, modified solar filters or solar filters from other manufacturers or for improper use of solar filters with telescopes.