Specifically, my bathtub is leaking into my laundry room, which is underneath the bathroom. Serious answers only, please, and thanks for the help.
1. you could call a plumber, there are services online to help you find one (like servicemagic)
2. you could ask at local plumbing shop or super store (good luck,) but you would essentially be trying to find someone to do the needed repair.
3. you could search online particularly in forums like this one where a knowledgeable person may ask questions to help identify the problem. (However they will not be able to fix it for you unless you happen to be nearby [unlikely])
It is possible that you don't need a plumber. The last several times I repaired a leak it was due to usage or maintenance issues around the tub.
For general information, if the leak is not under the drain area of the tub look for leaks through tiles. If the leak is constant regardless of use it is likely a leak in the supply pipes, hot or cold. If the leak occures during use on only some people or at some times and not others, look for someone who is leaving a curtain out or a door open. If the leak occures only when a shower is being used but not the tub look for a leak in the supply tube to the shower head or through poorly grouted tiles. If the leak occures during any use it may be caulking or something in the drain lines. If you fill up the tub and it is not leaking but begins to leak when you let the water out it is likely a problem with the drains. If the full or draining tub does not leak but begins to leak when the tub is very full look for leaks around the overflow.
To be sure to see when the leaks are happening instead of being delayed you may have to open up the ceiling of the laundry. If the leaks have been bad you will have to repair or replace the ceiling anyway.
Repairs to the drains are often done from a trap door in another room bordering the valve set of the tub or from below by cutting a hole in your laundry room. Repairs to the supply side of the system are most often done through the wall of the adjoining room to avoid disturbing the tiles/surround around the tub.