Does anyone know where I could buy a Saarinen white tulip base?  

eBay???
Is my best guess. I saw one listed as "Saarinen Style" earlier today. Not sure if that means its a reproduction base or just can't be authenticated because lack of marking/seller knowledge.

Yes
There was a new one on eBay that ended a few days ago.
You could probably find a vintage one for a reasonable price too.

I think you'd be better off...
with a "knock-off" unless your custom top is being designed to work with the way Knoll attaches the base to the tops, no?
Page pegboard and see if he has any in the "parts bin".

design integrity......
knoll or any authorized knoll dealer will not sell you a base to go with a top you are having built.
would eero saarinen approve of what you are doing ? no he wouldn't and thats why no one is going to sell you a base to create a fucked up version of his table.

A Burke/Arkana base will do...
A Burke/Arkana base will do you fine. Will be cheaper and more readily available.

What if....
You had a Knoll table and the base got "fucked up"????

Base
Where abouts are you based, I may be able to find you one in UK