You might try some pawn shops; just tell them the same thing you said in your post (you want a cheap plinking rifle), and ask for a "safe functioning" (not accuracy) guarantee, so if it fails to function you can bring it back. If they agree (some will not), specifically ask if it would be a cash refund (rare) or a store credit (useless unless they have several guns of the type you're looking for).
Other places might include sporting goods stores that sell shotguns and/or larger rifles (sometimes folks will trade-in a .22 on a new deer rifle or bird gun), or checking the newspaper classifieds.
Finally, I've had some success in asking co-workers if they know anyone with a cheap .22 for sale. Be careful in discussing firearms in a workplace; some places have a bordering-on-ridiculous ban on even discussing firearms due to "workplace violence" concerns, or bosses that are anti-gun to the extreme.