There is a free preview on Google books of the Mandaic dictionary by Majid Fandi Al-Mubaraki et. al. It is extremely limited compared to the out-of-print Drower and Macuch Mandaic dictionary, but at least there are other ways of getting access to this one than inter-library loan!
I must say that am extremely grateful to Mr. Mubaraki for sending me a copy of his dictionary and other Mandaic text and transliterations that he has made available. I would not have been able to make the progress that I have either in studying the Mandaic language, or in my study of Mandaean origins, without these resources.
For those interested in Syriac studies, Brigham Young University has online collections of texts at the Syriac Studies Reference Library and the Center for the Preservation of Ancient Religious Texts. Add those to the ones at Archive.org and it has to be said that it is rather impressive how much one can find available electronically.
Now if only there were audiobooks in Syriac and Mandaic, so that we could learn and study in the car and on the go…