Authority Information Access extension, which is added into certificates issued by a given CA, should contain a URL which points at a download location for the certificate of that said CA. This is described in section 126.96.36.199 of RFC 5280, in particular with this paragraph:
Where the information is available via HTTP or FTP, accessLocation
MUST be a uniformResourceIdentifier and the URI MUST point to either
a single DER encoded certificate as specified in [RFC2585] or a
collection of certificates in a BER or DER encoded "certs-only" CMS
message as specified in [RFC2797].
So that's either binary DER (not Base64), or a .p7b (which means PKCS#7, which is the old name for CMS). The raw DER-encoded certificate is probably better supported than the CMS object, therefore I recommend using it.