Is there an https download available of ubuntu ISOs?
There is currently no official HTTPS mirror, which can be quickly determined by reviewing the HTML source code of https://launchpad.net/ubuntu/+cdmirrors. The only HTTPS links are to other Launchpad sites, and any relative links obviously can't link to one of the mirrors.
However, you can download by torrent, which is actually relatively secure. Torrents include hashes for chunks, and through use in various P2P uses, have been strengthened from bad data injection and detection attacks, and so are quite safe.
While there may be unofficial HTTPS mirrors, they have the insecurity of being unofficial unless you get MD5 checksums from the official site and check these hashes with
Technically the official mirrors page only lists
Of course, regardless of where you download from, it's always a good practice to check hashes as in hexafraction's answer.