I have seen lots of little pots of creeping fig @ Home Depot and just the other day I was in Target and they had a hanging pot of varigated creeping fig and it was only 9 dollars. You could easily get enough "plugs" to fill your frame. It should be easy to find some.

But I believe I read in the book (in the link section) that while creeping fig is good to use, you can easily rot it, if you over water it in a stuffed form.

You can also try baby's tears.

In the tropics where I used to live, we used the creeping figs to cover our tall cement walls. One plant is sufficient to cover a large amount of space and so if given a large container to grow in, it would be able to grow large enough to cover your topiary frame. Make it 2 plants if you want to save time. Afterall these are slow growers.