Articles
This article contains the symbols I've found in a video from CMU Database Group, a YT channel from Carnegie Mellon University.
Basic Relational Algebra Operators
 σ (Selection)

Symbol: σ (Sigma)
Description: The Selection operator is used to filter rows based on a specified condition.
Example: σ(condition)(relation)  π (Projection)

Symbol: π (Pi)
Description: The Projection operator selects specific columns from a table.
Example: π(column1, column2, ...)(relation)  ∪ (Union)

Symbol: ∪ (Union)
Description: The Union operator combines the results of two tables, removing duplicates.
Example: relation1 ∪ relation2  ∩ (Intersection)

Symbol: ∩ (Intersection)
Description: The Intersection operator returns only the rows that are present in both tables.
Example: relation1 ∩ relation2  − (Difference)

Symbol: − (Minus or Difference)
Description: The Difference operator returns the rows present in the first table but not in the second.
Example: relation1 − relation2  × (Cartesian Product)

Symbol: × (Cross or Cartesian Product)
Description: The Cartesian Product operator returns the Cartesian product of two tables.
Example: relation1 × relation2  ⨝ (Join)

Symbol: ⨝ (Join)
Description: The Join operator combines rows from two tables based on a related column.
Example: relation1 ⨝ relation2
Additional Relational Algebra Operators
 ρ (Rename)

Symbol: ρ (Rho)
Description: The Rename operator is used to rename the relation or attributes.
Example: ρ(new_name)(relation)  ← (Assignment)

Symbol: R ← S (Assignment)
Description: The Assignment operator is used to assign the result of a query to a relation variable.
Example: R ← σ(condition)(relation)  δ (Duplicate Elimination)

Symbol: δ (Delta)
Description: The Duplicate Elimination operator removes duplicate rows from a relation.
Example: δ(relation)  γ (Aggregation)

Symbol: γ (Gamma)
Description: The Aggregation operator is used for aggregate functions such as SUM, COUNT, AVG, etc.
Example: γ(attribute, SUM(attribute))(relation)  τ (Sorting)

Symbol: τ (Tau)
Description: The Sorting operator sorts the relation by specified attributes.
Example: τ(attribute)(relation)  ÷ (Division)

Symbol: ÷ (Division)
Description: The Division operator is used to find tuples in one relation that are related to all tuples in another relation.
Example: relation1 ÷ relation2
Sources
"F2023 #01  Relational Model & Algebra (CMU Intro to Database Systems)" by CMU Database Group:
Addendum
I'm reading "E. F. Codd and Relational Theory (revised edition)" by C. J. Date, and he has a few more operators, but no symbols.