you could purchase a digital file and print your own. Found this on Etsy, don't know if they are the right shades but here you go:
Purple and YellowGold Printable Digital Paper by PurposelyDesigned
And GinaK Designs has a couple digital packs that feature purple and yellow:
http://www.shop.ginakdesigns.com/cat...?categoryId=40
By searching "high and low" does this mean you've checked stores like Michael's and Hobby Lobby? If not, you might want to check them out. Look at the large 12 x 12 pads of paper; if you find a collection that has a pattern you like check online to see if you can order that particular paper in the quantity you need. For instance, I found this pack at an online store, you could by the "collection" or purchase individual sheets of the specific design you like. There appear to be some purple and yellow; but not mainly purple; here's the link:
http://www.acherryontop.com/shop/com...ne/indie_bloom
One last suggestion would be to find a purple print paper you like and a yellow and find a way to combine the two in the invitation. Make the main body of the invitation purple but maybe design it with a yellow band around it that closes it. Or make the invite purple and the RSVP card yellow? Just a few ideas...Good Luck!