It depends what you want to test. Its growth behaviour changes depending on the substrate it is growing on, i.e. it tends to swarm as a mass cell on nutrient agar and branch out in seek mode on non-nutrient agar (I use 2% agar powder or cooking agar). Slime mould will grow on non-nutrient agar but will be more actively looking for food. See https://youtu.be/5tYKYpQzu6E at end when food is taking away its growth becomes more frenetic.
Nutrient rich agar also gets contaminated more quickly.
If you want to try an agar which contains nutrient but isn't visible or lumpy, you could try a mix of yeast extract (Marmite) and dextrose (energy tablets). I have used this as feed, which it loves, but haven't tried mixing it with agar - but I can't see why it wouldn't work.
Try 1 teaspoon of marmite + a third of a dextrose tablet per 100ml of water (recipe comes from Audrey Dussutour via Natalie Andrews). Though I suspect the agar will contaminate quite quickly.
Let me/us (the collective) know how you get on.
I have fed it flour before too, you could try mixing that with agar, it should disperse evenly.