i either look directly at the transparent tag or google it and click around the tumblr accounts that show up in the search! these are the first four that show up: 1, 2, 3, 4, and it looks like this one is listing a bunch of others. it's a lot of hit and miss to find something decent but it usually works out, especially if you have something in mind and can look through their tags for it (ie: coffee cup, illustration, etc)