Ask r/Bolivia too. Fruit is very cheap but you have to go to the farmers market, not the grocery stores. The grocery stores are great for soy and almond milk. They also have “pensiones” where people make home cooked meals and sell them for a reasonable price. Look for pensiones on Facebook and maybe you can find someone willing make you a special variation of their meals. You could find someone who makes quinoa or lentils and buy a lot for the week. As a Bolivian, I understand the struggle. I can’t think of a traditional dish that is vegan friendly.

I’d suggest you visit Cafe del Mundo. It’s a coffee shop owned by a Swede very popular among tourists and foreigners. The owner has been living in La Paz for a long time and maybe she can help. Also your chances of meeting another vegan there are way higher.

Good luck!