There are some 1" threadless forks sold, so look around. You'll also need to replace the headset & handlebar stem.
There are also still threaded forks sold, though you may need to cut and thread them to size, which adds to the cost. So Take your time and explore the total cost of both a threaded replacment, including the labor to cut and thread is needed, and a threadless fork, plus stem and headset.
As for sourcing, try the various internet sites, since 1" forks are in the minority now, and local shops are less likely to have much selection (if any).
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