I would go to the local auto parts store and get some "freeze 12" or some other R12 substitute with leak stop or whatever. Most of those things come in cans ready to screw onto the low pressure adapter.

Make sure whatever it is doesn't make it impossible to switch to R134a later.

If it were me, I would leave out the leak stop and just go for a quick charge if the leak is a slow one.

BTW, R134a is going up in price very rapidly. It used to be less than $4 per can at Walmart, as of last night it was $7.88 per can. Those are 12oz cans. I am hearing of local shops charging $60 per pound (16oz) because the price has gone up so much for them. Walmart's R134a price rises much more slowly because they buy in such hich quantities that the price increases hit long before they run low and have to buy again.

Clay