I usually get mine from ‘theappledorn’ on eBay. It’s currently priced at £11.49/litre with free postage. The only alternative to Mobil ATF LT71141 is ZF Lifeguard Fluid 5 which is manufactured for ZF by Pentosin in Germany, but it’s hard to get hold of in the UK anyway.
These are the only fluids you should use in your 5HP24A. Never use fluids that claim to be ‘equivalent’ or ‘recommended’ or similar wording. The friction modifier pack used in the fluid has been developed specifically to give the necessary static to dynamic friction characteristics required for the correct operation of the torque converter lock-up clutch when operating in its ‘continuous slip’ mode
The ZF document TE-ML11 provides other car manufacturer part numbers for LT71141/Lifeguard Fluid 5 but I expect that all of these will be more expensive than £11.49/litre
Phil