There are not many places where you can buy organ shoes, although dance shoes are often used instead and shoes of a similar ilk can also be worn. However, organ shoes may be obtained from the following retailers:

UK based

Church Organ World

Outside the UK

Organists KO

Organ Master Shoes

Tic-Tac-Toes

Please note that organ shoes purchased from abroad may appear cheaper than UK retailers, however there is likely to be a Customs Duty charge which will negate the cost benefit.

There are some interesting discussions on whether organ shoes are needed.

Organ Shoes – Do you really need them?

ABRSM – Organ Shoes

Volunteer Organists

Wiki