How to get there
The earlier you start planning your trip, the better. You can search for a long time how to get to Phnom Penh (the capital of Cambodia), but you are unlikely to find something direct and not for all the money in the world. But a $400 option to Bangkok and back will definitely catch your eye. In 2017-2018, Qatar Airways, UIA, Flydubai, Air Arabia had good options. Then you can fly Bangkok-Phnom Penhor Bangkok-Siem Rip ($60-100 round-trip), catch Air Asia sales.
Visa
Since November 22, Thailand introduced a visa-free regime for Ukrainians visiting the country for tourist purposes for up to 30 days. Now about visa to Cambodia. Visa to Cambodia can be issued upon arrival at the airports of Phnom Penh, Siem Reap, or at the checkpoint at the entrance to the country, the cost - $30/30 days. The visa requires 1 passport size photo or you will be asked to pay a $1-3 fine. You will also be asked to pay 100 baht to the officer at customs. Simple as that. Don't argue - you can't prove anything, it's official corruption.
On the Cambodian government website, you can make an electronic visa, which costs $37 for 30 days. However, there are cases when customs officers cannot find your e-visa in their database when crossing the border and ask you to pay the visa fee again. Even if your visa is rented out - it is very difficult to prove anything.