Where can I find a key for my old locks?

Heh, skeleton key mortise locks. You're into house restoration territory with this and need a locksmith that specializes in really old equipment. As these appear to be interior door locks, the ward and slot design will be quite simple, unless it was a boarding house where a little more security was required.

Per DMoore's comment --> Just so you know, looking for "Dexter Ward Lock" pulls up a lot of HP Lovecraft pages... Wooo! wooo! The couch cushions are probably haunted by some of the olde ones.

Dexter is now owned by Schlage, but has a long history of being in the door fixture business.

I'd also recommend trying places like this: http://www.historichomehardware.com/store/

We bought a home built in the 1870s which was updated in the 1930s. None of the interior doors had keys. Went to a local locksmith and explained the problem. They actually sell a set of common skeleton keys - it was around 5 if my memory serves and under $20. You just keep trying until one works.

Lot cheaper than a house call.

For this type of lock you would need to call in a specialized locksmith who can do that for you. They would need to cut a skeleton key for you. There was only one time where I went into a hardwood store and found a skeleton key and was able to fit into my closet door and actually worked. But I believe it worked because it was just a regular keyway that any skeleton key would work. I don't know if you would like to try that first.

One quibble with the other answers:

These are indeed simple warded lever-tumbler locks. The keys for them are properly called "bit keys". Warded/lever locks don't have to be lower security than pin-tumbler locks (safe deposit boxes still use lever locks, for example) but making them both secure and affordable is harder than with pin-timbler locks.

Skeleton keys were actually a kind of master key for these, cut to avoid as many ward variations as possible while still operating the lever(s) to permit retracting the bolt.

Yes, you can buy kits of skeleton keys and probably find one that will operate your lock. Odds are better if you bring the locksmith the measurements of your keyhole, so he can make sure you have the best-sized set.

If you really want to do this right, a locksmith can fit a key specifically to your door's lock. I'm planning to do that for my library door, as a historical touch. Ask for an estimate this can be a bit time-consuming compared to rekeying a modern lockset.

You can look at antique shops for keys they are super cheap and your key hole looks like mine. My friend had one of the common skeleton keys and came to my house one day and asked if she can test her Skelton key and it worked. She wouldn't sell me hers so I'm going to an antique shop which sells them for 3.50 to 7.50