Private Invitation Letter required for Ukraine Visa

Hi

I am a South African citizen flying to Zaporozhye, Ukraine in March and  busy applying for a visa. I am married to a Ukrainian citizen and we will be staying with her parents for vacation. I have all documents EXCEPT the Private Invitation Letter which is apparently required from the State Migration Service of Ukraine. I have done a lot of reading however  not sure of the safest and quickest way to get this document. Some say it takes a month to get it privately and others say you can organise it online fr reliable travel/tourist companies for a fee. Time is unfortunately not on my side.

Please could someone assist and explain exactly what is needed and how to get this Letter quickly.

Your assistance is highly appreciated!
Thanks

From here.  The Private Invitation Letter is required for business visits.  For a tourist visa, you need to supply "The documents confirming the touristic purpose of travel (original touristic voucher of relevant touristic agency/touristic agreement, booking or payment of hotel services, booking of excursions, two-way tickets, etc)."

Thank you Cynic. I have come across this already so I know what's required, however the difficulty is knowing who exactly to contact at the State Migration Service and which documents to send to receive the letter as quick as possible and what I'm expected to pay for it.

Based on my own previous expereince:
- your wife's parents can do it for you in two or three working days
- only your passport copy is required to send to them to ukraine
- this is based on the consul in Ukrainian embassy in your country, its up to him to ask you invitation or not, its their authority to give you visa with or without invitation letter but once you are married to Ukrainian citizen with certified documents by the embassy you don't need for invitation letter! but again everything in the consul hand!

invitation letter sample:
http://apartmentrentalkiev.com/img/visa_ua/invitation.jpg