All kinds of calculators. Inexpensive pocket scientific calculators from dollar discount store are super cheap, at prices like $1 and $3. Used calculators can also represent a good value. You can often find them for under $5 at thrift stores, as well.
Reviews and overviews will cover basic features/missing features, and the type of battery used.
- 99 Cents Store Scientific Calculator StudioArt SUPER Review
- Dollar Tree Jot Scientific Calculator 10 Digits