I welcome suggestions for OER materials at the NRS 1, Literacy Level, and slightly beyond. We've got Base-10 Blocks but not much money.
Number Talks are something teachers can use with students right from the beginning levels - especially when you start with unintimidating dots. They are completely free and open like you are looking for, but will require a time investment for the teacher to learn about them. In the same digest notification where I saw your question, I learned from Christin Smith that there is an online version of the book I have in my collection, Making Number Talks Matter: http://cdn.stenhouse.com/emags/0998-1/index.html
Here is one in action: https://www.youcubed.org/resources/jo-teaching-visual-dot-card-number-talk/
You might want to check out the upcoming LINCS virtually facilitated online course on OER Math resources. It begins on April 22nd. Here's the link to the announcement: https://community.lincs.ed.gov/notice/new-oer-course-open-math-open-resources-engage-adult-learners-21st-century-skills