I offered before in the Reconstituters thread to send you a batch of my Trace + Iron if you wanted to try it out. Offer still stands.
I got mine from eBay.
Nickel(II)-sulfat NICKEL (II) SULFATE HEXAHYDRATE 99,9+%- 250 grams
If you have any difficulty getting it I could spare a little (you don't need much, 100g is what I got and that's still a lifetime supply).
Add to 100ml RO/DI 1.13g of NiSO4.6H2O then dose into your ferts mix at 1ml/500ml giving you 0.0005mg/L in the bottle.
I missed a bit ^ I should have added that dosing the resultant mix at 1ml/10L gives 0.0005mg/L in the tank.