The phrase "to buy some pumpkin" means to purchase or get some pumpkin, referring to the act of obtaining or acquiring pumpkin for a specific purpose, like cooking or decoration. Full definition
Buy that pumpkin spice latte, or the lipstick that you aren't sure you will ever wear. (twiniversity.com)
I was messing around in the kitchen over the weekend, experimenting with some fall recipes and I ran out of the store bought pumpkin pie spice mix I had. (honeyandbirch.com)
Just make sure you do not buy the pumpkin puree which has the spices added to it. (joyofbaking.com)