I think its a beginners mistake to try and buy those colony ships.
To do so on large/huge galaxies is quite silly. After all, you start with 5000 and at most you can buy 4 or 5 ships and then it's finished.
What you should do instead, especially on those larger galaxies, is to first built a research lab, and tech-rush speed upgrades, possibly up to impulse drive. If you have Ion's you can make ships that go 4 units, if you research impulse, you can make your colony ships go a whopping 6 moves a turn.
Simultaneously, buy a couple of factories, maximize your industrial capacity and tax revenue, and spent it all on millitary. Just buy only factories! No ships.
If you do this, and buy a couple of factories on 2 or 3 worlds, by game turn 6 or 7 you are churning out 1 colony ship per turn average
But its very important not to get your hopes down! On my first games I found the AI overwhelming too, but after you get the hang of it and follow a few common sense guidelines, you'll find yourself smiling at beating down those AI's