If you don't have the game manual, you can download a copy here: http://www.ipdb.org/machine.cgi?id=2684

The switch locations and their part numbers are located on page 2-49 and 2-50.
If you don't know how to get into test menu, it's on page 1-14

...for parts, there is a thread on pinball parts shops with a sticky on the top of the forum.

good luck.

You probably will not even have to buy anything (if you already have a soldering iron + solder).

Check under the machine in the area where the ball drops underneath the playfield.. where the ball comes to rest before it is kicked up, there should be a switch that the ball rests on to let the game know it is there..
I suspect that switch has a loose wire on it that needs to be soldered back into place, OR the switch may just be bent and not recognizing the ball every time. You may have to bend it back into place.

Simple things first.

Welcome to the pinball world and Pinside.

Do you get the same problem when you hit the ball launch scoop, too? There is a switch by the slot machine kickout in front of the coil that is likely the problem.

Switch 58 (Slot Kickout) is intermittent. Enter switch edges test mode, raise the playfield, and operate the switch by hand. If it works every time, you just need to adjust the metal lever/whatever that the ball rests on so that the ball triggers it consistently. If it doesn't work every time, replace the switch.

Look at topics and click online pinball parts...all the good ones are there.