List of free rfc3161 servers.

rfc3161.txt

https://rfc3161.ai.moda
https://rfc3161.ai.moda/adobe
https://rfc3161.ai.moda/microsoft
https://rfc3161.ai.moda/apple
https://rfc3161.ai.moda/any
http://rfc3161.ai.moda
http://timestamp.digicert.com
http://timestamp.globalsign.com/tsa/r6advanced1
http://rfc3161timestamp.globalsign.com/advanced
http://timestamp.sectigo.com
http://timestamp.apple.com/ts01
http://tsa.mesign.com
http://time.certum.pl
https://freetsa.org
http://tsa.startssl.com/rfc3161
http://dse200.ncipher.com/TSS/HttpTspServer
http://zeitstempel.dfn.de
https://ca.signfiles.com/tsa/get.aspx
http://services.globaltrustfinder.com/adss/tsa
https://tsp.iaik.tugraz.at/tsp/TspRequest
http://timestamp.entrust.net/TSS/RFC3161sha2TS
http://timestamp.acs.microsoft.com

@Manouchehri great, thanks a lot, works perfectly. Now we can call our "time stamper" util with your load balancer.

I created a simple time stamp query http(s) client and sent a simple query to the servers mentioned in your list. Here are the simplified results:
Probably some of these URLs need a path (e.g. freetsa.org should be http(s)://freetsa.org/tsr).

http://timestamp.globalsign.com/tsa/r6advanced1: OK
http://timestamp.digicert.com: OK
http://timestamp.acs.microsoft.com: OK 
http://time.certum.pl: OK
http://rfc3161timestamp.globalsign.com/advanced: OK

http://zeitstempel.dfn.de: fail
http://tsa.startssl.com/rfc3161: fail
http://tsa.mesign.com: fail
http://timestamp.sectigo.com: fail
http://timestamp.entrust.net/TSS/RFC3161sha2TS: fail
http://timestamp.apple.com/ts01: fail
https://tsp.iaik.tugraz.at/tsp/TspRequest: fail
https://rfc3161.ai.moda/microsoft: fail
https://rfc3161.ai.moda: fail 
https://rfc3161.ai.moda/apple: fail
https://rfc3161.ai.moda/any: fail
https://rfc3161.ai.moda/adobe: fail
https://freetsa.org: fail
http://services.globaltrustfinder.com/adss/tsa: fail
https://ca.signfiles.com/tsa/get.aspx: fail
http://rfc3161.ai.moda: fail
http://dse200.ncipher.com/TSS/HttpTspServer: fail

@HeikoSchlittermann Could you please explain how your client works? Those results seem very wrong to me.

The following have stopped working for years:
http://tsa.startssl.com/rfc3161
http://services.globaltrustfinder.com/adss/tsa
http://dse200.ncipher.com/TSS/HttpTspServer

The following, as mentioned, had the URL wrong:
https://freetsa.org it should be: https://freetsa.org/tsr

The rest is working properly.
So something is wrong with thou simple time stamp client.