I get the cheap equine stuff from amazon - NAF is the brand I think, usually about £10 for 5 litres in one big bottle, https://amzn.to/3i88OFV or tate and lyles is available in big bottles too
I've used the big 7.25l bottles of treacle from costco before (when I had access to a trade account and someone already driving over there!) - most of the time it was only tate and lyle avaialble (but occasionally they had cheaper own brand stuff too) and that was always did a job.