PLpgSQL Library Management System Projects
Evergreen ILS (by evergreen-library-system)Project mention: Guidelines for high school library content and other questions... Any advice very much appreciated. | reddit.com/r/Libraries | 2021-02-07
As someone else mentioned Koha is open source so probably fits your budget! Another open source is Evergreen. https://evergreen-ils.org There’s probably lots of articles online comparing the two.
NOTE: The open source projects on this list are ordered by number of github stars. The number of mentions indicates repo mentiontions in the last 12 Months or since we started tracking (Dec 2020). The latest post mention was on 2021-02-07.