How many speeds? (2 in front, how many in back?) Chain width varies with number of speeds, and I just want to make sure we recommend a derailleur that's ideal for your chain width.

But speaking in generalities... Pretty much any indexing-compatible Shimano front derailleur made for standard double chainrings should work.

To give you a ballpark example... A Shimano Sora 3500 braze-on derailleur for a 9-speed (2x9) drivetrain and standard chainrings sells for about $20-25, brand new from Amazon. Sora's not a high-end groupset, but they're decent parts for the price. If you want something more midrange like 105, you can probably go there without spending a fortune (but you may have to go used or new-old-stock depending on the number of speeds in your drivetrain).