Some kind of water feature would be the best way. I have a small preform watergarden in the ground. I think it's only about 50-100 gallon. I also have 2 100 gallon stocktanks on the ground that I grow lotus, lilies and marginals in in the summers, and I'm amazed at the frogs that it's attracted. The tops of the stocktanks are 3' off the ground, plus I have 3-4' chicken wire around them (to keep the coons out), and frogs jump over that too! I have a green frog that goes from my stocktanks to my in-ground water garden all the time. I had the watergarden about 3 years, before the frogs came........but they WILL come! So my vote for you is to have a watergarden......big or small, and put plants in it that cover the water, to keep it cool for them, and for them to hide under. They like sitting on my lily leaves, under the shade of the lotus leaves! I love hearing them sing outside my window at night.
I wouldn't import any. Just let them come on their own. Mine seem to spend most of their time in the water or on the rocks I have lined the water garden with.
Gardenweb has a Ponds and Aquatic Plants forum, so you could check it out and read alot of the posts to learn about watergardens. I absolutely love my watergardens. They are easy to maintain and give me soooo much pleasure. And the frogs love them too! Feel free to email with any questions you might have about small watergardens.

If you have proper habitat for amphibians they will show up eventually.

Bullfrogs showed up in my pond all by themselves. They must have travelled more than a mile through the woods to get there.

Toads mostly come out at night so you might not notice them even if they are present.

A water feature will draw frogs from all over. It's a great idea to add one to your garden/property.

But toads don't spend time in water except to lay their eggs. They do like cool moist areas so providing shade and cover (overturned logs for example) is essential. I have toad houses but I have never seen a toad near one.

Another essential thing to try to garden organically. Don't use sprays or chemical fertilizers if you want toads in the garden (or frogs in the pond).