For those having trouble finding ships: don't forget that the game divides ships into three tiers or epochs: Mastery epoch (high armor, low shields, ballistic focus, usually brownish colour scheme), Expansion (blended focus, usually grey colour scheme), and Modern epoch (high shields, low armor, energy focus, usually light blue colour scheme). If you are unable to find a particular ship, you may be looking on planets that normally sell the wrong epoch ship. If I recall correctly, Hegemony bases sell Mastery, Sindrian Diktat sells Expansion, and Tri-Tachyon sells Modern ships.
If you're having no luck finding a particular ship, you may just be searching bases that have the wrong epoch ships. Try adjusting your search accordingly.