How can I get Review Units for my YouTube channel with below 100 subscribers?

It will not be possible to get review pieces from companies with just 100 or even a few hundred subscribers. Also, eCommerce websites won't send you review pieces but the manufacturers themselves will send you review pieces when you have a sufficient number of subscribers. I have had companies contacting me to review their products like a Spy camera, a microphone and recently I am expecting a Hotlink for Mac to come from the US. But these things come to you only if you have a sufficient number of subscribers to your channel. Aim for at least a 1000 subscriber count before even thinking about getting these things.