There are several commercial sources that you can purchase healthy or RA synovial fluid from, with complete patient data. Try bioscience.co.uk, you are able to select the particular cohort you want.

To be honest, I doubt you can rely on any synovial fluid being normal unless you have judged the clinical state of the source (human or animal) yourself. Goodness knows what might get labelled as normal. In general a normal synovial joint only has a film of fluid 50microns thick between the solid structures, with a few larger pools in nooks and crannies. In humans I never found a regular source of 'normal fluid'. I suspect if you want to set up a standard of a reasonable volume of fluid you can aliquot then it would be most practical to pool pig or cow synovial fluid from an abattoir. In the old days that used to be fairly easy but may be covered in bureaucracy now. There would be no way of judging whether such fluid should be considered 'normal' but I suspect that may be a false quest. Maybe all you really need is a good volume of standard that is not too full of inflammatory products?

There is an infrequent condition known as intermittent hydrarthrosis (IH). Its onset is usually seen in childhood or adolescence. It usyally affects the knees, causing considerable effusion of the joints. The synovial fluid may be mildly (no more than 5000 white cells/mL) or non-inflammatory.

Since I see the Institution where you are working, may be you could ask to your clinician colleagues to collect synovial fluid from their IH patients.

I don't know if such substance could be considered "normal" for your purposes, but IH synovial fluids are the closest and most abundant sources of it, to the best of my knowledge.