Can I use my HSA to purchase a new mattress?

Did you check with the HSA plan administrator?

I will! Thank you!

Amounts paid for a mattress or special bedding are qualified medical expenses when authorized by a physician to treat a specific medical condition, but only for the amount that exceeds the cost of similar regular bedding. Please retain the prescription or other order with your tax records, along with proof of the cost of regular bedding.

(You may have an issue, though, if your current mattress is simply old and worn, and would need to be replaced anyway.)

I doubt condition of current mattress would matter as only the amount above a replacement mattress is allowed.