How do I know if it is brake fluid? Where to get German brake cylinde

What work did the mechanic do, or what was he supposed to do?

Brake fluid has a thickish viscosity and is quite clear in comparison to motor or gear oil.  When you have some one your fingers it will be a bit slippery but not as much as the oils.

Take a look at the backside of your wheel to see if there are any signs of fresh liquid either spun around the rim and/or tire or any obvious signs of leaking from the wheel cylinder, bleeding valve and/or brake line fitting...especially in an area directly above the puddle.

Where exactly was the puddle in relation to the wheel?  How much fluid was there, and what sort of time frame was involved?

I think that description would fit gear or motor oil as well, 

Most probable is brake fluid.
Next on the probability list is brownish gear oil from the trans, which has a sulfur or rotten egg-like stink to it. Since your bug has a swing axle, it's possible to get it near the wheel by running down the axle if your suspension has not been lowered.
The least likely in that location is motor oil.

Not sure where to get a good German wheel cylinder... you will just need to shop around unless someone else know a supplier, or you can get yours rebuilt. Most of the ones available today are Brazillian or Mexican.

Paul