As Mary indicated, it is a large dose. Assuming you mean "mcg" vs "mg", this is roughly equivalent to 300 to 400 mg of morphine. You must be under a doctor's care.
You can look up the cheapest place to get your meds. That's what I do and it will give me the best price.
If you have no insurance there might be a discount card that you can print out and take to your pharmacy. My card saves me over $50 a month for my pain meds.
Just google, best prices on Phentenol 120 mgs. Make sure you put in your zip code.