My guys just sleeps in cotton sleepers, or now that the weather's warming up, sometimes just whatever undershirt he's been wearing through the day (if it's long-sleeved). I personally have never worried about fire-retardency, but if that's a concern for you, then I'm pretty sure everything sold commercially has to meet fire safety standards (i think?). Wool is naturally fire-retardant, so if you are looking into a more natural route- maybe look into that, good luck!
Sorry, i misread- you're worried about the chemicals used in fire-retarding things. In that case, go for a wool one, you could probably make one out of second-hand sweaters if you can't find one, but i'm sure there will be one somewehre out there on the www.
Sorry, i misread- you're worried about the chemicals used in fire-retarding things. In that case, go for a wool one, you could probably make one out of second-hand sweaters if you can't find one, but i'm sure there will be one somewehre out there on the www.