Hi Nick,
Are you looking for A or B stars in general or specifically ones suitable for use as reference stars?
If the latter, then for low resolution spectra the MILES stars (included in the ISIS database) have actual spectra associated with them or alternatively Francois Teyssier has generated a list of hot main sequence stars with low interstellar extinction which can be used (with caution) with generic spectra from the Pickles library. You can find his spreadsheet which also includes the MILES stars and has tools to help with selecting stars close to the target here
http://www.astronomie-amateur.fr/Documents%20Spectro/ReferenceStarFinder.xlsm
with more information in this post on the ARAS forum
http://www.spectro-aras.com/forum/viewtopic.php?f=6&t=821&p=3697#p3697
Cheers
Robin