[LintMan]

Here's a couple solutions to increase your supply on a ship in general:
- If you research Stellar Harnessing, you can build Solar panels then regenerate your supply a bit each turn in a system with a sun, so it's harder to run out.
- If you tech up the cargo tech tree (not too expensive), the supply bays get pretty sizeable.
- If you tech up the resupply tech tree enough (expensive!), you can get the quantum reactor and have infinite supply.

But if you really need long range and can't afford (or wait for) the quantum reactor tech, you can make a tanker ship. Just take any hull (a transport one is good, and you can use the cargo space to carry extra population) and fill it with as many supply bays as you can fit. Then create a fleet with your tanker and colony ship in it, and they will share their supplies and have a very long range. The problem then is that when you make your colony, your tanker is now way out there and will take a while to get back to where it is needed (if it has the fuel to do so, that is!).

I prefer to use the free 20K or so of free space on a colony ship to put in a cargo bay to carry more population - a level 1 cargo will let a colony ship carry 34M people instead of just 4M. I tend not to need longer range for colony ships, since I usually will colonize the closer systems first to create resupply bases as stepping stones outward.