Your best bet is to Angels Costume hire in London. They are the biggest costume hire company in Europe for Films / TV as well as for straight fancy dress hire.
They will have Gorilla costumes.
Either:
Contact: Richard Green
Angels The Costumiers
1 Garrick Road
London
London
United Kingdom
NW9 6AA
Tel: +44 208 202 2244
Fax: +44 208 202 1820
OR
Conatct: Sarah West
Angels Fancy Dress
119 Shaftesbury Avenue
London WC2H 8AE
Email: fdress@angels.uk.com
t:+44 20 7836 5678
f:+44 20 7240 9527
http://www.fancydress.com
Or both!
I hope that helps you.
Best, Isabelle