You can isolate DNA without Isoamyl alcohol. It is an anti-foaming agent and does not affect the actual out come of your preparation. Go for phenol-chloroform (50: 50) instead of phenol-chloroform-isoamyl alcohol (25:24:1) and follow the protocol as such. However, you should avoid vigorous pipetting during phenol-chloroform stage (50: 50). In case of plasmid DNA, you can directly shift to Li-Cl precipitaion method.
I have extracted as many samples with just phenol chloroform as phenol chloroform isoamy alcohol. There are meant be theoretical differences but in my experience both work perfectly well