To dress up your certificate a little bit more, consider some parchment or other patterned paper. Keep the certificate paper fairly light in color so your text has plenty of contrast. Also, darker papers or ones with strong patterns may interfere too much with graphic elements and colors you use for your text and graphics. A subtle laid or linen finish paper can give your certificate a touch of elegance without overpowering the design.
Most of these papers come in 8.5" x 11" letter size, a common certificate size. For smaller certificates, print multiple copies to a page and cut them. If your printer will handle 12"x12" scrapbooking paper you can increase your paper options greatly and have some really fun, patterned certificates.
Plain, Textured Papers
These parchment papers are lightly tinted and won't interfere with your text and graphics. Use them without borders or print your own borders.
Southworth Parchment Paper in gold, blue, ivory and other light colors, 25 lb
Staedtler Calligraphy Parchment Paper - 60 lb
Wausau Astroparche Parchment Papers
Scrapbooking Parchment Paper and 65 lb Card Stock in Letter size and 12x12 sizes
These linen and laid finish stationery and resume papers make nice certificates.
Southworth Linen Papers in white, ivory, gray, and other light colors, with 25% cotton content
Wausau Royal Linen Papers in white, ivory and other colors, acid free
Laid Finish papers from Southworth, Eaton and others in white, ivory, gray, some with 25% cotton content
Try some granite or other stone-finish papers for a visually heavier look.
Papers With Graphics
Stationery with printed borders can double as award certificates paper. Not all designs will work well in landscape orientation but there's no rule that says certificates can't be in portrait mode. If the theme works, use it.
Masterpiece Studios Stationery with a variety of borders and patterns
Great Papers Letterhead with pictures and borders and background graphics
Geographics Parchment Paper with Printed Certificate Borders in various colors