The official info from Fantastic Beasts and Where to Find Them mentions that they enjoy 'large prey'
The Acromantula is a monstrous eight-eyed spider capable of human
speech. It originated in Borneo, where it inhabits dense jungle. Its
distinctive features include the thick black hair that covers its
body; its legspan, which may reach up to fifteen feet; its pincers,
which produce a distinctive clicking sound when the Acromantula is
excited or angry; and a poisonous secretion. The Acromantula is
carnivorous and prefers large prey.
We know (based on mentions) that there are a considerable number of larger creatures living in the forest that they could be eating including deer, stoats, birds, foxes and sucking bugbears(?) as well as the odd thestral, hippogriff, centaur and unicorn. It would also, presumably include mundane forest creatures indigenous to Scotland such as badgers, pine martens, hares, foxes and wildcats, all of which would be potential fodder for an Acromantula colony.
Note that Hagrid doesn't think Grawp will have much trouble in the food department
"He can get his own food, no problem. Birds an' deer an' stuff"