Paske is the nearest biggest town to the Si Phan Don islands. The best way to reach Pakse is by bus. Regular buses and minivans are available from cities like Luang Prabang, Vientiane and Vang Vieng, which takes between 8 to 10 hours.
There are 4 bus stations in Pakse, which connects the town with other smaller cities too. The bus ticket varies between 20 to 30 USD per person depending on the city of departure. You can also get there by renting a taxi for about 200 USD. There are flights to Pakse from Vientiane, Bangkok, Siem Reap and Luan Prabang.
You can easily reach Si Phan Don islands by bus or songthaew from Pakse. The bus leaves around 8 in the morning and costs around 70,000 LAK. A cheaper option is to catch songthaews from Pakse's southern bus station. Take Ban songthaew to Nakasang if you are going to Don Det or Don Khone, and to Ban Khinak if you are going to Don Som. The fare to either of these places is 40,000 LAK.
There are also direct buses from Siem Reap, Phnom Penh in Vietnam which takes around 7-8 hours for the entire journey.