Yep, that's another option. Where I live they are very rare and the ones that are there are small and not many berries per plant. Gathering then from your own garden is very quick. Certainly quicker than the extra time you spend in the shop for them. And the ones in the shop are usually not ripe enough for my tastes. I like them sweet not sour.

Also buying 3 kg for making jam costs you a fortune. You can buy a jar of blackberry jam for less than €1, but buying that amount of fresh blackberries easily costs €10.