hi,
Open: C:\Program Files\LTC\LTspiceIV\lib\cmp [ your folder tree maybe different, but lib\cmp is the destination folder]
Find standard.jft folder. rename to standardjft.bak
Download the attached standard.txt file rename to standard.jft [ this is because the Site software will not allow .jft extensions]
Move this standard.jft to the C:\Program Files\LTC\LTspiceIV\lib\cmp
This will give you lots of nJfet models.
Section#3:
On your LTS circuit, use F2, then select the njf model, when the symbol appears on your circuit, right click on it, then select Pick New FET,
The 2N3819 should be there.
E
EDIT: try section #3 actions first, the 2N3819 may already be available.