There are hardly any train connections within the country, so the bus -- if available -- will be your only choice. Marshrutkas minibusses zip between cities much faster and often more frequently than buses. They cost a little more, but travel much faster and can be hailed anywhere along their route. If you flag down a marshrutka, it's customary to pay on leaving.
By taxi
By taxi
Taxis in Tiraspol are very common and are quite cheap. Be wary of scams however--make sure to negotiate a price before you get in the taxi. If you miss the last bus to Kishinev at 18.35, you can take a taxi from Tiraspol to Kishinev, which should not cost more than 30$.