Hi - from memory when i bought my Sumatra it was supplied with one clear plastic magand one metal mag - both worked without fault using Air Arms .22 field, however A/C guns in Hastings were the importer but I don't think
deal in them any longer although I may be wrong - Give them a call and ask to speak with Ray who I found to be a top man to deal with.
PELLETS: I have moved to the Isla of Lewis recently, however when I lived in Rugby Warwick Dive and Regulator Services had a stock of the pellets you are looking for.
Try https://www.divingcrs.co.uk/ or call 07977 410 748 as the owner was very much into Career rifles - it's been a long time (since lock down started) since I have been there so don't know if he still has stock.
Hope this helps - all he best - Nige.