Travelling to Kampuchea, you need an entry visa. This is necessary for all passport holders. As a novelty, you can ask for a visa in electronic way, the necessary form is available at Thailand Foreign Ministry web site, as follows:
Using this way of presentation of application, you are obliged to pay a fee relating to:
USD 20 (basic fee) plus this extra fee, relating to USD 5. Apart of this, you can ask for the respective entry visa at Cambodian Embassy, Consular Section, abroad (for instance, in Warsaw, Poland), or at any border crossings (in case of European tourists, directly on the airports).
However, when visiting Cambodia from Thailand territory, you cannot get it, in a moment of your return way to Thailand, Thailand visa again, and you are forced to use air transport. Provided you have the intention, during your stay in Thailand, to visit Cambodia, too, it would be moresuitable to ask for Thailand multiply entry visa. So, you could avoid this problem, mentioned above.
Embassy of the Czech Republic
Information and warnings on travelling abroad is to be found HERE.