How can I obtain crystal-clear glycerin?
You can buy high-purity glycerin from numerous vendors online. High-purity grades will be as crystal-clear as you can possibly get. 

For instance, check out the premium grade glycerin product from Sigma-Aldrich which should be suitable for your needs but is quite expensive. 

There are less expensive (but pure enough for your needs) options, so be sure to explore the other grades available from other vendors. For example, you can secure a gallon of Kosher USP food grade vegetable-derived glycerin for $14.99 from Amazon. This product (or similar) is adequate for your needs at a better price point that lab-grade reagents.