If by synthetic you mean a non-soap based bar cleanser, they should be pretty easy to find.
Walk into any local grocery store/walmart and any bars that say "body bar" or "bath bar" will most likely be detergent based instead of soap based.
2 things you can look for to identify detergent based soap
1) It doesn't say "soap" anywhere on the packaging
2) The ingredients are listed on the product
Usually "real" soaps list their ingredients as well, but it's not required by law. Synthetic (eg, detergent based) bath bars are required by law to list their ingredients.
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