This information might be now outdated but here goes anyway.
If you ever go to Copacabana (the main port leading to Isla del Sol), try to get some soles, bolivianos, dollars, anything, from an ATM (in Puno, La Paz, wherever you’re coming from!). When I went to Copacabana I found out the touristy fishing village on the shore of Titicaca had two ATMs. One works only with Bolivian cards. The second works with international cards, but it had been blocked by a cement wall and hadn’t been working for the past couple of months. The tourist office girl was very polite: “Yeah, I know, a lot of people complain about that problem.” Stressed? Never. You get used to them good ol’ Bolivian ways pretty fast.
There’s a funny story involving me having to return some sausages to a lady vendor in some market, in order to have enough dough to buy a ticket for the last bus to La Paz… but I’ll tell that one later.
Copacabana is still worth it, however!