This is the most reliable source for fresh trimmings. Many butchers sell beef fat for around $1.99/lb , and some may even give it away for free or at a very low cost if you ask.
For immediate cooking use, pre-rendered tallow is widely available at major retailers and online: Grassfed Beef Raw Fat - White Oak Pastures where can i buy beef fat
You can find beef fat in several forms—raw trimmings, suet (high-quality kidney fat), or pre-rendered tallow—depending on whether you want to render it yourself or use it immediately for cooking. This is the most reliable source for fresh trimmings