Connect Bidet with 1/2 inch supply line to 3/8 toilet line
You can use an ordinary 3/8 tee, with a 3/8 outlet on the side and then add a supply line that has 3/8 on that end and 1/2 on the end attached to the bidet.  Here's a link to one example:
Brasscraft 3/8 to 1/2 inch FIP faucet connector