Hi Adrian,
Sorry, can’t help with a supplier, but can help with a conversion chart. Have a look at the following site …. http://ipmsstockholm.org/ …. it has a conversion chart that covers Federal Standards, Humbrol, Tamiya, Xtracolor, Polly S, Gunze Sangyo, Model Master.
The site is one that should be in everyones Favorites folder, it also shows which paint numbers to mix for a particular colour.
Also there is a wealth of hints, particularly handy for those building dioramas, galleries etc. – A must look site.
Cheers – Kenny