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What are you exactly trying to do? If you only want the ink, I would advise you to search for it in ebay: the price is reasonable and you will get the best results with the commercial version. The 8 oz bottle goes for around 20 bucks.
If you want to actually make it, it gets more complicated. Most organic and natural fluids glow under a blacklight, so I would experiment with everything that is around my house (milk, whine wine, even saliva, etc) to see what gives me the best results. Hopefully you will find something and I think that it could be fun to experiment :)
I have a theory of something that may work, but I haven't tried it... maybe you want to try it yourself: I'm 97% sure that vitamin A and 80% sure that vitamin E will be visible under a blacklight. Both are liposoluble, so you could try pills and dissolving them in clear, non-colored acetone. Already dissolved presentations are available, and they're mostly inexpensive; however, the commercial ink may be even cheaper.
I hope this will help you, and if you try it I would love to know which results you got :)
Try a few of the following materials:
Bodily fluids show up too, but I dont reccomend those.