The viza (with Z) is the name of the entry ticket to the territory of Kazantip.
You shouldn't confound it with a visa (with S) you need to enter Ukraine. Moreover, to enter Ukraine, if you have a European passport, you do not need a visa. kazantipvoyage.com gives you all the information to get your entrance ticket to Kazantip.
All the answers to your questions as for the entry ticket to Kazantip with kazantip voyage.
online purchase of the entry ticket Kazantip ? recommended, you pay less...On an exceptional basis, Kazantip sometimes starts online sale of vizas (pass) on its official site. Vizas of discount price are available but in limited quantities in the context of the promotion. Keep yourself informed.
purchase of the entry ticket upon arrival day ? The number of places is unlimited (Kazantip is never sold out for the territory of Kazantip is very large) and usually the purchase is made locally in numerous currency exchange offices set up by Kazantip.
You can now book your stay at Kazantip safely without the risk of not getting admission ticket because the number of entry tickets is unlimited and purchase on site is possible in numerous currency exchange offices set up by Kazantip.