either have a seperate table that just connects the two
unit_suppliers
unit_id,supplier_id
with the primary key being a combination of unit_id
and supplier_id
or have a suppliers
field in the units table that contains a comma separated string of the supplier ids for that unit
For the first case, to get suppliers for a unit the sql would look like
SELECT * FROM suppliers WHERE id IN (SELECT supplier_id FROM unit_suppliers WHERE unit_id='$UnitID')
($UnitID
being a php variable containing the units id)
For the second you would do
SELECT * FROM suppliers WHERE find_in_set(id,(SELECT suppliers FROM units WHERE id='$UnitID')) <> 0
Then in your form have some way of selecting multiple suppliers for a unit and insert into the database accordingly