I would suggest sending it to Air Arms for a full service.
Anyone looking to do it themselves will find information on a strip down on the Air Arms Owners Club site.
Spares - all 'o' rings are available from bgaltd (Julian) on the same site, look on the for sale thread.
I can't help with individual sizes, probably easier to buy a set from Julian and have them posted.
L/H Pro-Target MK111 Burris Timberline 4.5-14x32 Ballistic Plex
L/H TX200HC Burris Fulfield II 3-9x40
L/H TX200 GinB
Why is dyslexia so difficult to smell?.