Apart from going down to the beach and picking it up yourself, there are a few different places and it's worth shopping around - bargaining the price down a bit is quite normal and expected. Wood does seem to be expensive here as it's not really used that much in the building industry.

For driftwood tabletops, logs, and farmed poles: Sing Chang Wood Factory (abt 143.5 on no.11)
Late afternoon negotations will probably involve some drinking of alcohol - actually that can happen in the morning too. Reasonable prices, but you do need to bargain here, and you may be able to get him to throw in some offcuts lying around. (Chen Lao Ban)

processed timber - boards: Jhong Sying Rd (drive to the end of Hsin Sheng, turn left into Jhong Sying - it's about a kilometer up the road on the left next to a seven-eleven)
Processed driftwood boards as well as tabletop pieces. A bit on the expensive side and definitely need to bargain. Limited range of sizes.

Supposedly if you go left at the same 7-11 and drive down a couple of hundred meters there's another place that's quite good, but I haven't been there.

processed timber - boards, posts: 66 Taian village, Beinan shire. Tel: 0932 663 572
Reasonable prices and a good range of different sizes - kind of tricky to get to - it's off the no. 9 near an elementary school. Best to call and get directions.

If anyone can add more detail with the directions, contact details, etc, that would be nice, or some info about other woodmills in the taidong area would be great too.