Here's some studies that are free to download:
Sitt: 100 Etudes op.32 (1st position)
Paganini: 24 Caprices for solo violin No. 1 / No.2 / The whole 24 are availble free to download here, but they're zipped files (unzip them with WinZip or similar program to open)
Kreutzer: 42 Studies or Caprices are available free to download here, except that they're zipped files.
Rode: 24 Caprices: no.1-5 / no.6-10 / no.11-15 / no.16-20 / no.21-24