In a side lock, the Lyman Great Plains Rifle is the best bang for the buck.
If shooting patched round balls, make sure the barrel says "Great Plains Rifle" - if shooting conicals, look for "Great Plains Hunter". (different twist rates) These rifles have two barrel wedges and quite often, one will have pressure on it and one won't, so it falls out easily and gets lost. Take the barrel out and peen the slots closed just enough to grip the wedge.
Sometimes the squeaky wheel gets replaced...