Do you mean that you don't have a downpipe from the roof gutters?

How close is the downpipe? For example if it's on your next door neighbour's house (and lose to your boundary) could you negotiate with them to set up a water butt each?
Otherwise you could set up a waterbutt and put out buckets when it rains, then fill the waterbutt from the buckets - a bit of a palaver though. I've put my pots on bigger pot saucers/ trays than I would normally - quite a lot of water collects in them and I rarely have to water.
If you do sort out a waterbutt take a look at the slimline ones I have a 100 litre one in my terrace yard - it wouldn't do for someone with a proper garden but is plenty for a yard.

What are you growing and are you mainly in ground or pots? Front garden or back or both? Trying to get an idea of your garden water demand.

If you have a downpipe then if it's a square one you can cut it and put in a block and small piece of pipe that can feed into a water butt, you could have three water butts doing it this way, or if you don't have a down pipe then for the long term it may be worth having one put in more so if you are on metered water.