Where can I buy a MagSafe charger for my iPhone se

Assuming you have the newer iPhone SE, it supports standard Wireless Qi charging pads.

https://support.apple.com/en-us/HT208078

And you can get those at just about any retailer that sells mobile phone accessories. Qi charging pads are an industry standard that most newer iPhone and Android phones both support.

https://www.apple.com/shop/accessories/all/wireless-chargers

If you are looking for a magnetically attachable charging cable for your iPhone SE's built-in Lightning port (similar to a Mac MagSafe charger) that would require a 3rd party adapter easily available on shops like Amazon. It would not be under the name MagSafe, and would not be certified by Apple as MFI compliant. You'd simple need to search for something lighting: magnetic lightning port cable.

You can buy it online from Apple. See: MagSafe Charger - Apple

Also note the following from that site:

The MagSafe Charger makes wireless charging a snap. The perfectly aligned magnets attach to your iPhone 12 or iPhone 12 Pro and provide faster wireless charging up to 15W.

The MagSafe Charger maintains compatibility with Qi charging, so it can be used to wirelessly charge your iPhone 8 or later, as well as AirPods models with a wireless charging case, as you would with any Qi-certified charger.

The magnetic alignment experience only applies to iPhone 12 and iPhone 12 Pro models.