Using /give you can give yourself bucket 0-6 which is empty bucket, milk bucket, cod bucket, pufferfish bucket, salmon bucket, tropicalfish bucket. Absent from the /give interface is Water bucket and Lava bucket. This appears to be a bug because you can clear all buckets using /clear @a bucket, which also clears water and Lava buckets.

Edit: I found that water and lava are bucket 8 and bucket 10, but bucket 7 and bucket 9 are skipped causing the appearance they do not exist. Misleading to be fair.