I’m trying to find more of this stem that I found at Trader joe’s but they couldn’t tell me the name of it.
This is a cultivar of Sweet William (Dianthus barbatus). There are several cultivars with similar green "flowers", including 'Green Ball' and 'Green Trick.' Sweet William is closely related to the carnation (Dianthus caryophyllus), which is why the stem looks a lot like the stem of a carnation.
Dianthus barbatus 'Green Ball' (image source)
Dianthus barbatus 'Green Trick' (image source)
To get more of it, you can order online from someplace that sells floral supplies - look for a place that is geared towards DIY floral arrangements for weddings or other events, rather than someplace that only sells pre-made bouquets. You may find them by searching "green trick flowers," "green ball flowers," "green dianthus flowers," etc. If the supplier website is organized by categories, these will be in a "filler flower" or "greenery" section. Or ask a local florist; if they have this in stock to use in their flower arrangements they will probably be willing to sell you a few stems.