There are some e-shops selling minerals - and also vivianite. I would recommend to order some cheap and not attractive sample since it would cause much pain to destroy some nice crystal...
Since I´m not a geologist, I´m not really familiar with minerals, so I was wondering if is there any other forms of iron-phosphorus mineral which have different "crystalline grade"?
I would like to make some experiments to test the dissolution of iron phosphate minerals that have different "crystalline form" (poor crystalline to high crystalline).
Well, I am not a mineralogist but there is another Fe-containing mineral called strengite (FePO4 · 2H2O) with crystalline structure different from vivianite. It is more rare and its aggregates are rather small (x mm) but samples are available in e-shops for circa 20 EUR. There is one place in my country where crystals of this mineral are found. I give some links:
I suggest you contact some authors who have recently published papers on vivianite and ask if they have samples they could share with you. It would be especially useful to pick someone who has studied properties relevant to the research you intend to do. There are several recent studies in Geochimica et Cosmochimica Acta. You might also look in Economic Geology.
What kind of experiments are you planning? I have a couple students currently experimenting with vivianite formation and degradation.
The type of experiment will determine the form of vivianite you need. You can get it as a powder ground from clay from Natural Pigments and Kremer. I am a pigment specialist so I can't help you if you're looking for crystals: crystals don't make good pigment.