You can also get 4 in 1 chuck keys. 4 different sizes, plus the 4-way comes in a couple of different sizes to fit small keyed chucks and large keyed chucks. The chuck maker and model number ought to be on the chuck itself. Genuine Jacobs chucks always have info engraved on them. If you can find that info you should be able to find out what chuck key size to get.
The chuck should be removable. See if there is a slot in the spindle the chuck fits into (it is part of the drill press itself). The slot will be a few inches above the chuck and goes all the way through the spindle. You will need to have a tapered drift or something similar to get the chuck out. Almost anything that will fit into the slot and go in over halfway through the spindle should work. Slip the tool into the slot and raise the spindle, HARD. The tool in the slot will stop against the spindle housing and apply pressure to the end of the drill chuck arbor. It should fall out, so be ready to catch it. If you have the actual correct tool, a tapered drift, you can tap it into the slot with a hammer, forcing the chuck arbor out. Once you have the chuck out you can take it to Sears or Northern Tool or a hardware store or maybe Lowe's and match a chuck key to it. Before reassembly make sure to clean the spindle hole and chuck arbor spotlessly clean, then ram the chuck back into the spindle. You need to use a bit of force to make sure the taper seats tightly.
