generally, they are put on by having the handle ground down, threaded, then have the bolt handle screwed on. lots of selections out there, just have to do a google search for bolt hadle, tactical bolt handle, oversized bolt handle, etc.
i know midway and brownells have some economical ones, some could cost much more. depends on what you can find and what you want to pay for.
there's always the "super cheap" way:
LOW BUDGET .22 TRAINER - tactical/o.sized bolt knob
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