Translators needed

Here's a (possibly ignorant) question. Much has been made (and rightly so) of the lack of attention given in various quarters to literature in translation. It's important that awareness of global literature increases for English-speaking readers. But what about books originally written in English and translated into other languages?

I have searched in vain for Spanish translations of Ellison's Invisible Man, or anything by Walker Percy or Annie Dillard. Joan Didion, David Markson, Stephen Dixon, and Wendell Berry are similarly absent, although Nabokov, Pynchon, and Vonnegut seem to be pretty-well represented (thankfully).

William Gass' In the Heart of the Heart of the Country is translated, but unavailable. There is a Spanish version of Richard Powers' In the Time of Our Singing, but these two seem to be the only translated works of the above authors.

Flannery O'Connor's stories (as well as Wise Blood) are translated into Spanish, but not Mystery and Manners.

Are these decisions entirely market-based? Is it up to each publisher to decide whether a work is "worthy" of translation into a second language (Spanish in this case)? Granted, all I did was look up certain authors on Amazon. Is there another resource I should be using instead, a better way to discover which works have been translated? (I find it very difficult to believe that Ellison hasn't been translated into Spanish.)

I am sure there are many possible explanations and responses to the issue (I'd love to hear them! Perhaps I just need to learn more about how publishing works?). But is there a way for the average Spanish-speaking reader to obtain copies of items not found in a general web search? Or is it true that these authors simply have not yet been translated into Spanish?