Please supply the actual organization of your database, I dont have enough information to provide an aswer with your description.
However, as your database is now, I don't know how (and if) it can be done. You don't explain in details the structure of the database but I fear that it is not normalized.
If the main information the db handles can be divided among Customers, Suppliers and Products, the "normal" way of working consists in creating three tables: one for data about the clients, one for data about the products and one for data about the customers (in a totally normalized database Clients and Customers would probably be stored in the same table, but let's keep things simple and allow for two tables).
To create the relationships you need two junction tables: one will maintain a many-to-many relationship between Customers and Products (a product has more than one customer and a customer buys more than one product), and a second will maintain a many-to-many relationship between Suppliers and Products (once more: a supplier can supply more than one product and some products can be supplied by more than one supplier).
Under their simplest form, these two tables consists in two columns: one receives a copy of the primary key values from one table (say Suppliers) and one copy of the primary key values from the table Products. The same process is repeated for the tables Customers and Products.
These copies of the primary key values are named foreign keys (FK). Each FK can be repeated several times in the junction table but each pair of foreing keys must be unique, each pair in the junction table is also the (composite) primary key for the table.
Let's suppose that we have two suppliers with the IDs A and B and 4 products with the IDs 1, 2, 3, 4. Supplier A sells products 1, 2 and 4 while supplier B sells products 2 and 3. The junction table creating the relationships would look like this:
Code:
Supl. Prod.
-------------
A 1
A 2
A 4
B 2
B 3
The same principle is used for the relationships between Customers and Products.
You can then use Access to define the relationships: