As Mr. Knuckles mentioned there are plenty of web vendors who supply sports exhausts for the classic 500. There are two types of fitment. Exhausts for the 499cc engine and exhausts that fit the 594/652cc engines. So make sure you buy the right one for your engine size. Stainless ones will of course, cost a lot more.
Then, biggest question is single pipe or dual pipes. Unless you have a very highly tuned engine the sports exhaust won't make a huge difference to power output / acceleration but will add some noise. Always hard to get web clips of exhausts to listen to. But there will be some around.
I have an Ansa twin pipe in stainless. Link here to insta for video sound.
For UK, some vendors to assist you:
AutoBella Parts ~ Here at AutoBella Parts we are classic car parts specialists providing high quality parts for your classics & modern cars. We mainly specialise in classic Fiat, Lancia, Alfa, Ferrari parts. Get in touch with us to enquire about any part! Fiat 500 Classic Specialists.
autobellaparts.com
Euro vendors:
Spare parts for Fiat 500, Fiat 126 and Fiat 600 - from the specialist Axel Gerstl - since more than 30 years. We carry the widest range of bits for the Fiat 500 D, F, N, R, Giardinera, the Fiat 126 (BIS), Abarth, Bianchina in our online shop.
webshop.fiat500126.com
The best Sports Exhaust for your classic Fiat 500 | Easy order from stock | Worldwide shipping | Best service | FD Ricambi.com
www.fdricambi.com
www.passione500.it
My favorite tuning company, could spend a fortune here they have a great selection:
www.dangelomotori.it
Let us know how you get on!
Cheers
Rob