The normal filesystem location for the LPD queue directory is:

Answer: /var/spool/lpd

Explanation:
From http://www.pathname.com/fhs/pub/fhs-2.3.html#VARSPOOLAPPLICATIONSPOOLDATA:

/var/spool/lpd : Line-printer daemon print queues (optional)
The lock file for lpd, lpd.lock, must be placed in /var/spool/lpd. It is suggested that the lock file for each printer be placed in the spool directory for that specific printer and named lock.