I use the heavier plastic of ziplock gallon size bags. I cut them at the seams and pierce them several times. Each bag serves to line two 14-18 inch hanging baskets. I did this with strawberry plants about five weeks ago and I have been pleased with the results.

I used cheaper, thinner grocery bags last year and some of them didn't hold up very well. The garden center recommended using their clear plastic plant saucers without poking holes in them, but those cost $1 or more.

if your potting mix is draining to fast then putting a plastic liner wont help too much. you need to look at your potting mix. my hanging baskets with the coir bottoms get watered once a week.

Terrybull: What are you using for potting soil? In Southwest Ohio, I would be concerned about only needing to water once a week in the summer. I am using Al's 5-1-1 in my baskets this year. Last year I used a quality nonsoil mix with composted bark and peat. When it got very hot and dry, I had to water every other day. I don't think that's unreasonable. Four weeks into this season, I think the 5-1-1 has already proved it's worth. We've had unseasonable cold and record breaking rainfall. None of my hanging coir-lined baskets are soggy, and they are growing well.

my baskets are pretty big 22 inches and 16 deep. but my mix for them is 4 parts shreded bark, 2 parts peat and one perlite.

Sounds a lot like 5-1-1, which is what I suspect the original poster plans to use.

no its 4-2-1

So, I guess you are saying that the little bit of extra peat keeps your soil mix moist for a week in an unlined coir basket?

please read carefully 4-2-1 not 5-1-1