Loop for multiple select statements - postgresql

Question

I am a bit confused on how to deal with looping on multiple select statements using PL/SQL for PostgreSQL. I have written a function that works as expected for multiple select statements that each return a single row (see code below).
I have a middle table (called bridge_table) that breaks up many-to-many relationship between tables 'stock' and 'client', where one client_id can match up with multiple stock_id.
How/Where do I add in the necessary LOOP syntax to grab all the necessary stock_id's for a client_id? Is this a scenario where a cursor might be useful?
Thanks ahead of time!
CREATE OR REPLACE FUNCTION my_func(
a_id INTEGER,
b_id INTEGER,
c_id INTEGER)
RETURNS BOOLEAN AS $$
DECLARE
current_time TIMESTAMP;
new_inv INTEGER;
inv_id INTEGER;
inv_q INTEGER;
current_quant INTEGER;
still_true boolean := true;
BEGIN
current_time := clock_timestamp();
SELECT i_id INTO inv_id
FROM bridge_table
WHERE bridge_table.client_id=c_id;
SELECT inv_quant INTO inv_q
FROM bridge_table
WHERE bridge_table.client_id=c_id;
SELECT quantity INTO current_quant
FROM stock
WHERE stock.i_id=inv_id;
IF current_quant < inv_q THEN
RAISE NOTICE 'STOP';
still_true := false;
ELSE
new_inv := current_quant - inv_q;
UPDATE stock
SET quantity = new_inv
WHERE i_id=inv_id;
END IF;
IF still_true = true THEN
INSERT INTO log_table (log_id, u_id, client_id, time)
VALUES (a_id, b_id, c_id, current_time);
ELSE
RAISE NOTICE 'TRANSACTION WILL NOT OCCUR';
END IF;
RETURN still_true;
END;
$$ LANGUAGE plpgsql;
Edit:
Table DDLs
CREATE TABLE stock
(
i_id INTEGER PRIMARY KEY,
name VARCHAR(120) NOT NULL,
quantity INTEGER NOT NULL
);
CREATE TABLE clients
(
client_id INTEGER PRIMARY KEY,
name VARCHAR(120) NOT NULL
);
CREATE TABLE bridge_table
(
client_id INTEGER REFERENCES clients (client_id),
i_id INTEGER REFERENCES stock (i_id),
inv_quant INTEGER NOT NULL
);
CREATE TABLE log
(
log_id INTEGER PRIMARY KEY,
u_id INTEGER REFERENCES users (user_id),
client_id INTEGER REFERENCES clients (client_id),
time TIMESTAMP NOT NULL
);