Extensive collections of the entire series (2016-2021 editions), including workbooks and teacher books, are hosted by groups like First English For All Children and Enjoy English 2-11 .
You can find and download the workbook for grades 5-6 by M.Z. Biboletova (Enjoy English) through several educational repositories and community archives. Depending on the specific edition you need (older combined 5-6 grade or newer separate editions), here are the most reliable sources: skachat tetrad po angliiskomu iazyku 5-6 klass biboletova
Teachers often share compiled UMK materials, including zip files of workbooks for multiple grades, on Infourok . Components of the 5-6 Grade Set including workbooks and teacher books