Where can I find this manga: Ginga Sengoku Gun Yuuden Rai, also known as The Heroes of Galaxy Wars or Thunder Jet? I've been looking online for a place to buy this, but cannot seem to find it anywhere.
You should check out the YesAsia website.
It has several volumes of the manga you requested for sale for around $12 (€9). These releases are only in traditional Chinese and Japanese. As for English copies, I do not believe there are good sources to buy from as of now.