The wiki has a list of suppliers (feel free to update!):
Looks like Goss is the CIL rep in the UK, maybe they can help:
On 1/24/2013 9:14 AM, Felicity Shelley wrote:
> Dear All,
> Does anyone know where you can buy small volumes (1L tank) of 13-labelled methane from? In the past we've had it from Sigma but they're having "compliance issues" and so can't say when or if they'll ever be able to supply it.