Some time ago the EPUB versions of the Jataka Stories translations edited by E. B. Cowell disappeared from the internet. Now they are back, reformatted for a better reading experience.
Check them out on readingfaithfully.org here.
This is a full translation. Ken and Visakha Kawasaki have an anthology available for sale from Amazon that is basically an edited and modernized version of parts of this text. While they did a good job, one shortcoming of their edition (aside from the fact that it is only an anthology) is that they have fully integrated the canonical verses into the prose commentarial story. This isn’t a big deal, but it does erase the distinction between what is considered canonical and what is not. The language in the Cowell translation is actually not that old-fashioned sounding, so it is worth checking out these ebooks before purchasing the anthology from Amazon.