That first suggestion wouldn't go with Mission Style. Abstracts like that were not compatible with the "arts and crafts" naturalistic ethic of the Mission movement. Most artwork from that era was either "plein air" realistic landscapes, scenes from romantic legened or designs that looked very much like Japanese woodblock prints. They almost always used plain, almost hand-made looking wide dark oak frames, no glass and colored or no mattes. Here is one example of "plein air":
http://www.art.com/products/p10032122-sa-i847930/diana-reineke-poppies-en-plein-air.htm?sorig=cat&sorigid=0&dimvals=0&ui=1a5ddda277a54ee7b353b84bdc443128
This artist's work below would be perfect but is very costly -- but you might find something similar for less somewhere:
http://stores.ebay.com/ANITA-MUNMAN-20th-CENTURY-FINE-ART
here are some Japanese landscape woodcuts:
http://www.art.com/asp/search_do.asp/_/posters.htm?ui=1A5DDDA277A54EE7B353B84BDC443128
I also think the paintings of Gustav Klimt look wonderful with Mission colors and decor:
http://www.art.com/asp/search_do.asp/_/posters.htm?ui=1A5DDDA277A54EE7B353B84BDC443128
As you can see, most of these links are at www.art.com. You can get virtually any size of any of their thousands of art prints and most car be ordered on a canvas transfer. You can do a search on "arts and crafts" and it will show you some other art from the Mission era such as designs by Charles Rennie Macintosh and Frank Lloyd Wright.
But another source people often neglect is small galleries and art fairs in your own city. You can often buy original landscape or still life paintings and prints from the artists themselves that will be unique and may be actually valuable some day.
I also recommend browsing through books on "Mission Style" in bookstore home design or architecture sections. This will give you a good idea of typical artwork of the era.