What are some affordable vendors to support mass texting to multiple groups of customers (for a nonprofit)?

I have a nonprofit client that engages hundreds of college-aged students each year as mentors for young students; they also engage 1,500+ adult-aged volunteers in supporting these students. They would like to explore a way to send mass texts to these groups such as: - Weekly tips for their college-aged mentors who are working with kids; - Monthly inspirational quotes/life tips/etc. to the thousands of alumni who served as mentors when they were in college; - One-time thank you messages (potentially with an image) to the volunteers (e.g. on the day after they volunteered) There does not necessarily have to be a donation component. In the future, they also might be interested in a text-based giving program; however, for now, they are just looking to use this tool to strengthen relationships with these important stakeholders. The only program I've researched so far seemed too expensive. I am looking for a very affordable means for communicating regularly with supporters by text. Thanks!