Although this site hasn't been updated in a while, its a good resource for comparing coins depending on their size.
https://sites.google.com/a/themagicianscoin.com/www/the-coins/medium-coins
You can go for authentic Chinese coins, or for ones made by magic suppliers. Keep in mind the cheaper you go the poorer quality coin you will get. If you looking for the best available, imo they would be the ones made by Todd Lassen. As with all his work, they are incredibly high quality, and are available in dollar and half dollar sizes. They will not be cheap, and you will have to get in contact with him to have them made. Depending on his schedule and coin availability, it could be a while before you receive them.
Another good option would be East African dimes, they are the same size as Kennedy halves and have a hole punched in the center. A good option if you are looking for a more "authentic" coin to use.
As for Euro coins, there a bunch that would work, so I will recommend the half dollar size ones I like to use. U.S. Barber Half Dollars are my choice for half dollar sized coins, and the coins I use with them are French Napoleon III 10 Centimes, or as I like to call them, French Barbers. They are 30.2 mm in diameter, not very expensive ,especially for worn ones, and are thin, just slightly thinner than a Barber Half. The big positive of these, is they become very soft and quiet without the need for artificial wear, and still retain their detail fairly well. If you do coin magic that requires transfers of multiple coins, stacks, or edge work that benefits from soft coins, such as those found in Kainoa Harbottle's Coins On Edge, these would be an excellent choice for those. One thing to note, is the edges of these coins are very smooth, the milling is nowhere near as sharp as a Kennedy Half, which as a result can cause some people to have difficulty with certain palms, but you get used to it after some practice.