Just joined, so I am not sure how to do this.There is quite a supply of spare parts available to fix many of the mechanical problems in the old H&R revolvers, like replacing the plastic yoked mainspring rod, cylinder lever spring, sear, trigger spring, etc., but unfortunately the parts you are looking for will be difficult to find & if you do they may not be as good as what you have. Like others have said, you would probably be better off selling yours as a parts gun & using that money to buy one in good condition. 999's in excellent to good condition are for sale by the hundreds on line. I have a dozen H&R's from the 40's, 50's, & 60's and other than the plastic yoke main spring issue in the 60's all have performed flawlessly. They made a decent "workingman's" gun that was affordable. The 999 was their top of the line, so if it has been shot to the point it has the wear problems you describe, it's time to retire it.
I recently bought a H & R Sportsman in very good condition made in 1947.. Appears to have not been fired, or fired very little. Upon receipt, I fired 9 rounds with no problems. A few weeks later I fired 2 or 3 rounds and it locked up..Really out of time!. Cannot cock hammer, or pull trigger. The cylinder lock is not going into the notches on the cylinder. I cleaned and oiled but problem remains. Any suggestions will be welcomed. Don