Norfolk Island Pine
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I am assuming your plant came with a care tag and this is where you have got your information about monthly fertilization with a 20-20-20 fertilizer. Tags that come with tropical plants or houseplants (in Michigan) are general recommendations and are not specific to locations.
Houseplants in Michigan homes (unless they are grown under plant lights) receive far less light fall through spring. They are not actively growing and thus require no fertilization. What they do require is less but thorough watering (water so it runs out the drainage hole) and do not water again until the soil about an inch down feels dry to the touch. A monthly trip to the shower for a room temperature spray cleans the dust and some insects off of the plant. When you fertilize harmful salts can build up in the soil. Watering thoroughly so that the water runs out the bottom (discard this drain water) is the best way to prevent salt build up.
Regarding fertilizer, the tag is recommending a balanced fertilizer so a fertilizer with mostly equivalent numbers will work. For my houseplants that do not bloom, like your Norfolk, i use fertilizers that have a higher nitrogen or first number. I most often fertilize at one quarter the recommendation and do it every time I water but only beginning late spring through summer. If this is to remain a houseplant remember fertilization will increase it's growth and it may outgrow your space.
If you want a specific fertilizer and cannot find it I would recommend checking with your local nursery or greenhouse. They usually have a wide selection. Again though, your plant in Michigan does not need fertilization at this time.
I've attached a link on your plant for more information.