You won't have any trouble finding slot machines in any Japanese city, because there are slot parlors near the train stations and in most shopping districts. Slot machines are called pachi-slo in Japan, and they are part of an industry fueled by manufacturers who regularly roll out new models. When the new models hit the parlors, the old ones become outdated and eventually find their way to foreign markets. In Japan, the machines accept tokens, but you can modify them to accept quarters. The exact method depends on the manufacturer of the machine you have.
Insert the key that came with the machine into the slot in the door, turn it through 90 degrees and open the door. Plug in the machine and turn on the main power switch, usually located just above the coin hopper.
Insert one of the tokens that came with the machine into the coin slot on the outside of the door and observe its path as it drops down. The machine must be on when you do this or it won't accept the coin.
Insert a quarter and observe what happens. Depending on the type of machine you have, it may fall out of the front of the coin mechanism or out of the bottom. This happens because the quarter is 1/32-inch smaller than the token. To get the machine to accept the quarter, you have to cover the hole through which the quarter fell or make the raceway inside the mechanism slightly narrower.
Turn off the machine and unscrew the coin mechanism from the inside of the door. Disconnect the wires and turn it around so you can see the raceway.
Cover the hole through which the quarter fell, if there is one, with cellophane tape. Mechanisms with holes that prevent the user from inserting smaller coins are common on machines with horizontal coin slots. You may have to lift one of the plates forming the track to get the tape under it. Make the raceway narrower by gluing a strand of 18-gauge metal wire to one side of it with hot melt glue.
Adjust the position of the bottom rail in the coin mechanism upward if the machine has a vertical coin slot. Loosen the screws holding the rail with a screwdriver and reposition them in their slots so that the raceway is slightly narrower.
Reconnect the wires and attach the coin mechanism to the door. Turn on the machine and drop a quarter into the coin slot to make sure it slides down the raceway and that the machine registers a credit. Adjust the position of the wire or screws if the quarter gets stuck.
Close the machine door and lock it after you have made the adjustments. The machine won't operate normally until the door is locked.
The details of the coin mechanisms vary from machine to machine and from manufacturer to manufacturer. If you have a manual for the machine, consult it for the best strategy to modify the coin mechanism.
Pachi-slo machines are intended for private use as games of skill. Using them for gambling is illegal.