An underquilt is like a sleeping bag that attaches to the bottom of your hammock. By hugging the bottom of your hammock it prevents the insulation from getting crushed and is one of the keys to staying warm when hammock camping in colder weather.
We don't yet sell our own brand of underquilt (stay tuned!), but there are a number of great manufacturers out there making underquilts that work perfectly with our hammocks. The two that we recommend reaching out to are:
Like Trek Light Gear, both of those are companies are great, hardworking small businesses that love to help their customers - if you reach out to them directly they can guide you to the best underquilt for your needs and even customize it to match the exact temp rating, carry weight, etc. that you're looking for. If you let them know you've got a TLG hammock they should have no problem directing you to a matching underquilt.