URN for ECDSA signature algorithms (including hash algorithm) The URN form ECDSA signature algorithm is urn:nist-gov:ecdsa.

But I am not able to find a named URN for algorithm SHA1withECDSA or SHA256withECDSA.

Up to now I've found the xmldsig URL for this algorithms: http://www.w3.org/2001/04/xmldsig-more#ecdsa-sha1 and http://www.w3.org/2001/04/xmldsig-more#ecdsa-sha256

And I have the option to generate the URN with its OID, according to RFC3061: urn:oid:1.2.840.10045.4.1 and urn:oid:1.2.840.10045.4.3.2

I could even create my own version of its URN as something like urn:nist-gov:ecdsa-sha1 and urn:nist-gov:ecdsa-sha256

But, is it a named standard URN for signature algorithms SHA1withECDSA and SHA256withECDSA?
According to the IANA registry at https://www.iana.org/assignments/urn-namespaces/urn-namespaces.xhtml, there is no nist-gov URN namespace.  Outside obscure antisocial protocols like RFC 2802 there is no reason to try to use this URN namespace.  You appear to have gotten these URNs from a defunct internet-draft.  If you're using XML signatures, as you appear to be doing, then you can use use the URI http://www.w3.org/2001/04/xmldsig-more#ecdsa-sha256 like RFC 4051 specifies.

(But consider throwing XML signatures in the garbage—if not XML too.  Just sign binary blobs with a fixed signature scheme baked into your application; using XML signatures is asking for trouble.)