Welcome to the forum.
The biggest reason that most people experience cold in their hammock is because they don't have enough insulation beneath them, and air movement robs them of their warmth.
There are really two approaches to fixing that.
1 - Underquilts - quilts that are filled with either down or synthetic insulation that are suspended beneath the hammock, and provide insulating 'loft' to retain your warmth. These are generally the most comfortable option, but are the most expensive.
2 - Pads - using a sleeping pad beneath you is a very cost effective method of retaining warmth. They are typically inexpensive, but can be a little tricky to stay on top of while laying in the hammock.
If you want to look at quilts, have a look at
JRB and
Speer. They both have good commercially available solutions.
If you want to look at pads, even the typical blue walmart pads insulate well. Have a look at the
Speer SPE, though. It is designed to help you stay on top of a pad while in a hammock.
Again, welcome. Let us know how it goes for you.