Which buck converter can be used for 120 V AC to 5 V AC rated at 5 amps?
The easiest option is probably to use board mount AC to DC power supply brick.
Such as those made by Recom, XP Power, Meanwell, or CUI.

One example would be...
IRM-20-5 Vin=85V AC to 264V AC, Vout = 5V DC, Iout = 4A max.
The whole thing is only 2.06" x 0.94" x 1.07" and costs only $11.00
https://www.meanwell.com/Upload/PDF/IRM-20/IRM-20-SPEC.PDF
https://www.digikey.com/product-detail/en/mean-well-usa-inc/IRM-20-5/1866-3042-ND/7704667

If you insist on doing it yourself 
Keep in mind that the peak output voltage of the isolation transformer needs to be at least a few volts greater than 5V.
1)  Because you are going to 100se 1~1.5V in your bridge rectifier.
2)  Because the input is AC you are going to have ripple at the output of your rectifier.  Including ripple, your input must remain greater than 5V for a normal buck converter to give you a steady 5V output.  If you don't leave yourself a lot of voltage headroom then the holdup capacitor on the output of your rectifier will need to be huge.

A common 10:1 isolation transformer with 120V RMS in will give you 12V RMS out (17V peak).  Run the transformer output through a full-bridge rectifier with a large holdup capacitor on the rectifier output and you will have something near 15.6V DC.  You can then use just about any 3.5A buck converter at that point.

Some Usefull Stuff for Finding Converters.

If you pick a converter chip from Texas Instruments you can use the free TI Web Bench tool on the Texas Instruments website.  It will generate a schematic and list of components given some power supply criteria.  With the click of a button it does like 100 designs and presents them to you so you can evaluate the trade offs.

If you pick a converter chip from Linear Tech/ Analog Devices you can simulate it using their free LT Spice program.

Distributor websites like Digikey.com or Mouser.com allow you to search for chips based on parameters.  It makes finding what you need a lot easier.

https://www.digikey.com/products/en/integrated-circuits-ics/pmic-voltage-regulators-dc-dc-switching-regulators/739