A reprint of Frank Kidson's Traditional Tunes is available from Llanerch, mentioned above by Nutty.  They have had to wind up the original business, but the mail-order side at least is continuing as Llanerch Press, at a new address:

Llanerch Press
Penbryn Lodge
Pandy
Cribyn
Lampeter
Wales
SA48 7QH

Their website has not so far been updated, but contains a full list of reprints available:  http://www.llanerch-publishers.co.uk/

Dave Herron (Eckersley) of the Traditional Song Forum has compiled a useful list of folk song books in print in Britain and Ireland, an updated version of which can be seen at the  South Riding Folk Network  site:

David Herron's Chapbook