I run into this problem all the time. Go further distances. In my area I have utilized comps over 20 miles away but the trick is to label the appraisal rural. Fannie Freddie accept comps at greater distances if the appraisal is labeled rural.
Also, add as many long distance comps as possible in areas of similar economic condition. If still limited comps utilize listings or pending sales! Also please advise the appraiser to address the reason for the long distance with a detailed addendum. Appraisers like to write a sentence or two but this will not suffice for a complex appraisal like yours. Especially in our current depreciating enviroment.