If you can't see the oil from the leak spot - that means it is leaking where you cannot see. A leak detector will have a hard time locating a leak you cannot access. This becomes a circular argument with time.
Virtually all refrigerant leaks are in the evaporator. Why? Because that is the wet area prone to corrosion. The evaporator is under the dash in a spot that takes ~ 10 hours to disassemble. Then a new evaporator can be purchased from the dealer.
Other leaks (not in the evaporator) can be spotted by looking for oil. They also make an oil that fluoresces under black (UV) light that is added into the system. But, if the leak is where you can see, the oil from the leak is obvious.
But, I never want to be accused of stopping someone from buying a new toy. If you want a freon leak detector, expect to pay ~150.00 (new) for one that is a decent instrument. If you insist on paying less, make sure you can return the instrument that does not work as intended in a reasonable amount of time.
My (free) advise: buy some R134a with leak stop and add a couple of cans. If that works, buy a case of R134a at a discount price and add some when needed. Good luck!