Here are suppliers for a few of the things you might need for your masked ball costumes. Please note that unless specified, we cannot personally recommend any of these suppliers (if you have used any of them and can endorse them, let us know!)
http://www.baththeatrical.com (bonus of actually being in Bath)
http://www.costume-hire.co.uk/ (pricier, but again, based in Bath)
This corset pattern has been recommended to us: http://www.haabet.dk/patent/Waist.html
Abakhan Fabrics – I have used this website to buy fabric and they have several stores throughout the north west of England / North Wales.