Jawel in Coventry made me up some paint 'cellulose' last year to the manufacturers paint code and got the job to supply the thinners etch primer and primer in the process for there excellent customer service, delivery by courier was very reasonable also.
I have found a supplier local to me who will mix batches as small as 250ml but are a bit costly.
Jawel supplied 2.5 lt for about £60 but my local just charged me £40 ish for a litre