Smoke detectors are easily available at hardware
stores and Lowe’s and Home Depot and hardware
stores and maybe lots of other stores among other places. There are codes for where you want them. Top of the stairs and so on.
Easy to diy
Search for “smoke detector locations.”
And don’t forget about a CO (carbon monoxide) detector.
And replace the batteries
every spring and fall to avoid that annoying chirp.
Not hard, and worth the effort.