Here are some links:
- Rhetorica ad Herennium Passages on Memory
- Passage from Rhetorica ad Herennium
- What Are Some Good Memory Books to Read?
You might be able to find a copy in a library. Here are some other copies, starting at USD $1:
- Rhetorica ad Herennium (Kindle)
- Rhetorica Ad Herennium: The Reader’s Digital Edition
- Cicero: Rhetorica ad Herennium - (English and Latin Edition) (The sidebar has a used copy for $14.)