If you want to buy one secondhand, good luck. They're very expensive with prices ranging from $1200 and up. For some situations, samples will suffice, but some people say samples do not capture the true nature of the TR-808. So wether or not samples are good enough is completely subjective.
Another option for 808 sounds are various hardware and software "clones" which set out to reproduce the 808 sound as faithfully as possible. Roland, who originally created the TR-808, has never made a dedicated TR-808 clone themselves, and will most likely never produce this legendary machine ever again. They have, however, offered 808 samples in many different products for well over a decade now.
"We be going boom like an 808"- Blaque