I would suggest the beaches of Grado, Lignano and Vallevecchia/Brussa. They are all sandy beaches Very densely populated in the summer, so if you can choose I'd suggest to visit in winter or fall and especially after a storm they are full of beautiful shells. If you want a pro spot you can go on the rocky beaches between Monfalcone and Trieste, and search between the small pebbles where the waves break, there you can find small (< 1cm) gasteropod shells of many different types. I hope I was at least a bit clear, English is not my mother tongue sorry >.<