Mike, Irina is right on the name and website of the main companies that travel from Borispol to Krememchuk. But unlike what she said is not possible through the internet to book any bus ticket, you'll also find during the holiday season that reservations are complicate(if possible at all due to the fact that the bus is fully booked from origin and they are not prepared to take reservations for half the ticket.
Finding the offices of Autolux and Gunsel at Borispol is easy, as soon you pass the customs check point you will find a lot of taxi drivers offering you a very expensive ride. You just turn to your right, there is a couple of cash machines where you can get local currency at a better rate than in the other counters.You just turn right again and there is with an orange sign the counter for giunsel, try to remember that you are in ukraine and the name is written in cyrilic. Further ahead in the same corridor , you have the office for autolux(again written in cyrilic)
They do speak very basic english, and they will try to get a bribe in the form of an alleged reservation fee which I always pay because is still a lot cheaper than the taxi. The bus drivers , as soon as they see that you are foreigner will want their cut on the bribe and will ask also for a bagage fee that I also always pay as it is a matter of fact that every time that we travel to visit the family in Kremenchuk we bring loads of presents and I think that it is fair to pay that money. As a guide , the price of the ticket last time we travelled was 50 UAH pp, reservation fee 10UAh and the bagage was 30UAH. Still far of the 500UAH that was the best fare that we could get by taxi.
Taxis are more expensive in Kiev than in Kremenchuk, it has been always the case that booking them from kremenchuk make perfect sense but remember to negociate always the price as life in ukraine is hard and money in short supply and they try their best to make the most of any customer.If you have problems finding Gunsel or Autolux outlets in Borispol you can ask the taxi drivers and they will be happy to help you with no problem as people in ukraine are very polite. If you want any more specific information about the town , do not hesitate to ask as I am married to a girl over there and we do travel there quite often.Enjoy your trip!