Thermometer:  Temperature readings are an important part of entomology for fly-fishers since water temp directly impacts the timing of a hatch.  It's also a tool that many fly-fishers already have, so it was not inclued in the kit.  There are many great thermometers on the market, find one you like...

Sorting Tray:  The sorting tray is a "nice to have" item if you are going to take several samples and want to pick among the insects.  For general use, the insects can be viewed and pulled directly from the seine.   Any light-colored tray with a small lip works fine (think plastic lids, cups, soap dishes, etc)

Denatured-alcohol:  This can be picked up at your local hardware, pharmacy or larger grocery store.  You'll find pint and quart containers, either of which will last you a long, long time !