I have never seen 8% saline available, but 10% is out there usually in 15 ml vials. So you could use 10%, but when I had to do it toget a good sputum specimen, I found it harsh.

To make 8g saline:
Option #1: I did some math, and you can approximate 8% saline by mixing 4 5ml (or 5 4ml vials or 7 3ml vials) vials of 7% saline with 1 15ml vials of 10% saline in a sterile container (like a sanitized prescription bottle) this will give you 35ml of 8.3% saline.
Option #2: Make your own 8% saline. Use non-iodized, and not sea salt which naturally contains iodine. Use grams/ml for best accuracy. Dissolve 8 grams of salt in 100 ml sterile water in a sterile container. (For a while 7%saline was in short supply and I did this out of necessity. )

Store the solution in the refrigerator and use within 3 days. Use a sterilized dropper, syringe dosage cup or dosage spoon to measure 3, 4, or 5 ml saline into your nebulizer. (Actually I did this once and marked the neb cups on the outside with a permanent marker- exactness is not mandatory here.)