It sounds like the DBT therapist assumed you were already outfitted for fishing, with all/most of the supplies you feel you need? Simply tell her/him you aren't currently equipped to fish.
I have seen many ppl fish with a makeshift pole or a very, very inexpensive pole, fishing line, a sinker, a hook and a worm. It depends upon the nature of the body of water and the terrain, as to whether or not one needs to be able to cast the line or not, etc.
If you don't have what you need and cannot secure what you need, just let the DBT therapist know this and choose an alternative activity. The idea is to do something you enjoy before the DBT meeting.
If it's all too much, too overwhelming, please inform your therapist(s) of this. It's important they know. they may change your treatment plan.

WC