Your string of lights has a large "dead" section AND you can't find the replacement bulbs AND you don't know the location of the fault--but you hate to trash the entire string. You can probably fix them without too much hassle!
I have bags of replacement bulbs scattered through my garage, but the creativity of the light producers results in light sockets of many shapes and sizes, none an exact fit to the string under repair.
Step 2: Repairing an Individual Light
Fixing an individual light is easy. Let's assume the green socketed lamp on the left is bad.
Bend the wires down from the bad lamp (green socket) and good lamp (red tip).
Pull the glass lamps straight out.
Place the good lamp (red tip) into the green socket.
Bend the legs. You now have a working light that will fit your defective string.
Step 7: Finding the Bad Spot
If the bad string of lights is embedded in a tree, you might consider a light repair tool (I can't personally vouch for this--reviews are fairly positive).
These light strings are generally "groups of 50 in series." Each group represents about 120 ohms.
Use an ohm meter with pointed probes (multimeter on ohm setting).
Mark the start and end of the "dead" section using masking tape.
You will start with 50 dead lights. Unplug the lights. Go to the center of the bad area and remove one light (I use a tiny flat blade screwdriver to pry the bulb assembly from the socket). Press one probe (we'll call it probe A) against the right side of the lamp socket. Press the other probe (probe B) against one of the flat blades of the plug. If there is no reading, move probe B to the other blade of the plug. If there is no reading, the problem is to the right. Do the same process to the left. You should find continuity on one side of the lamp socket and no continuity on the other. Move the masking tape marker to this point, pointing toward the bad area. You have narrowed the problem to twenty-five bulbs.
Repeat this process and you will be down to twelve.
Repeat again and you are down to six; followed by three.
In theory, each bulb has a shunt, so filament failure should not cause a blackout.
Shunts sometimes fail. Filaments often burn out.
I have also seen (many times) a situation where the bulb (on the test bench) is good and the socket is good, but together they don't work. If the little wires on the bulb are very tight against the bulb, they may not make contact with the socket. Pull the wires out (like the second photo), insert the bulb and all should be well.
Have a brilliant holiday season!