One way to go is with what is called a lateral arm. It is a horizontal arm that mounts on top of a tripod, and allows your camera to be extended out over a subject. You normally would put your current ball head on the end of the arm. You may at times need to add a counterweight to make things work out. Here is an example:
There are also tripods that have a column built in that is designed to tilt over to the horizontal postion. These are made mostly for macro use, and will work for shooting straight down. Again stabilty may be an issue, and some form of counter weight may be needed. The Gitzo Explorer series of tripods is one example of this kind of tripod, but they have discontinued their aluminum version, and the carbon fiber version is very expensive. You may be able to find a used Gitzo aluminum Explorer at a more reasonable cost.
Another way to go is if you have a tripod with a center column that can be reversed or that you can mount a head on the bottom of the column. That can allow you to put the whole tripod over your subject and shoot down on it with the camera underneath the legs. This kind of setup can sometimes be awkward to use, but if you have the right tripod, you may be all set with no cost.
I have a Foba ASMIA tripod head which allows for pointing a camera beyond straight down. If straight down is -90 degrees from the horizontal , the ASMIA goes to about -105 degrees.
I actually usually end up screwing a short piece of their display tubing into the other end
and weight it to damp vibration/counterbalance the camera/head
I also have a gitzo explorer that works fine for stuff away from the studio.
I also have a couple of copy stands of various sorts but almost never use them anymore (the upright often makes lighting "interesting"). if you want to try that route you can usually find an enlarger for free and replace its head with the camera head of your choice.
