I want to purchase pre-threaded bobbins. What size and where can I purchase for kenmore model 1231?

SOURCE: instructions for theading a Sears Kenmore Sewing machine model 158.14000

all machines thread the same. TTN tension,take up, needle.
Beginning with the spool, guide the thread through the thread guides to the tension assembly. Pull the thread through the tension assembly and test it to make sure the tension is working correctly.
(To do this test,adjust your tension setting to normal or medium or 5 or something in the middle. Raise the presser foot and pull the thread through the tension. It should pull easily!! Lower the presser foot and pull the thread. It should pull noticeably harder.) If it passes this test, continue up through the take up lever,then down through the thread guides to the needle. Make sure your needle is fully inserted and turned the correct way.( If you load your bobbin case from the left side,the needle rule is normally FLAT to the RIGHT is RIGHT. There are some exceptions. If you have a newer style drop in bobbin or your bobbin case loads in the front then the rule is FLAT to the BACK.

To wind the bobbin, guide the thread around the thread guide on
the left top of your machine, then to the bobbin winder. Wind the thread around the bobbin several times, place the bobbin on top of the bobbin winder, push it against the wheel and press the foot pedal. It should stop winding when the bobbin is full or you take your foot off the control

Sewman7

SOURCE: I have the Singer 4562 and need more bobbins

The machine does take the class 66 bobbins.

SOURCE: will not pick up bobbin thread

Unusual for this machine. Check to be sure the needle is inserted with the flat side to the back of the machine. If it is, remove the needle plate, bobbin and bobbin case. Turn the hand wheel slowly toward you and see if the point of the hook is above or below the eye of the needle when the hook point is aligned behind the needle. If the eye is above the point of the hook the machine is out of time and best taken to qualified technician.

SOURCE: Singer Sewing Machine model 9224. Bobbin thread

If you turn the handwheel, can you see a metal ring rotating around the outer edge of the bobbin case? If not rotating, broken gear or timing belt off.
If that ring is turning, remove the needle plate and bobbin case. As this ring now rotates, you will a finger on the ring that will pass behind the needle from right to left and it should catch the thread from the eye of the needle. If this finger, the "hook" passes above the eye, you have a timing problem.

Let me know.

SOURCE: Purchased a HomeStyle sewing machine Model

Check the needle to see if the flat side is facing to the back of the machine. This is usually how the needle should be inserted.

Thread the machine with the presser foot up.

Hold onto the thread tail at the needle and hand walk the needle down and then back up to pull the bobbin thread up.

Take both threads under the presser foot and to the back of the machine before you begin to sew.