Try yellow pages for timber suppliers, or try some of the boatyards down on the thames, they may well have some small lumps of teak lying around for nowt! Just as an aside, if your boat is an older one, you may find the teak plugs are imperial, not metric, you may find it difficult to find imperial size plug cutters, I did, but ordered and obtained a set, from a good tool suppliers, they are available.
You may find <A target="_blank" HREF=http://www.antiquetools.co.uk>Tony Murland</A> very good for this type of tool.
I bought a set of Japanese Pull Saws from him (actually part of leaving present from last company) and he is most helpful and a fund of information. The saws are brilliant too!
I get all mine sent to me from K J Howells,they do them from 6mm upto 14mm in 2mm incriments phone 01202665724 just tell them how many you want and they will post them to you. www.k.j.howells.dial.pipex.com hope this helps.....
As ccscott says - check carefully the actual size, I have a set of imperial plug cutters but most people only seem to have the metric type.
I'll try Tony Murland in the future - he's only a few miles away.