Any robust F’ or male E. coli strain may be used with M13. We prefer ER2738 (#E4104S) or NEB Turbo (#C2986K, #C2984). Cells may be made competent or purchased that way. For the most complex libraries, electrocompent cells must be used (eff. ≥109 transformants per ug M13). See our protocol for making electrocompetent cells in the Ph.D.™ Phage Display manual or purchase from NEB’s line of electrocompetent cells.