I'm impressed Becky, we need more women who can work on plumbing. Seems like a lot of DIYers who post here are women doing complicated jobs.
If you removed the trap and found no plunger, then it is down stream somewhere. Tub traps are usually 1 1/2" and shower traps are 2" although 2" is roughed in sometimes anyway so there is an option to go with.
However you may be able to open up a cleanout downstream somewhere and chase the plunger to the cleanout with a snake assuming the snake won't grab onto the chain. Otherwise it may have to get chased to the city sewer main. Since you are handy, go to home cheapo and buy you a hand snake. Good luck.