Plant bracing above the bubbler!

So I cant get my hands on any rockwool or any of that- so Im relying on simply a cut out styrofoam cup to hold my plant's roots into water.

That is- I cut a hole in the middle of the cup, and stuck the plant right down the middle. The roots are growing well, the plant is growing leaves- but I know it cant stay in this tiny hole forever, or can it? What should I do?

Its set up like (maximize your window):

\topofcup/
|plant|
|--s-|
| rrrr |
| rrrr |
|_ r _|

s = stem
r = roots
The stem is sticking in that [--s-] which is just a piece of circular styrofoam that fits snug in the cup. (The cup is not as skinny as I typed it out to be)

I know eventually the stem needs to get thicker, Im sure. Or can I keep it like this? What other medium is good that isnt rockwool that I dont have to order online- say, i could get it at home depot or wal mart or the pet store or something? I just need a good brace for my plant is all.

Why cant you get rockwool? You dont have any garden centers or nurseries by you? No internet or hydro shops??Ebay has it for like 100 cubes for 12 bucks deliveried

ive read that just suspending the plants from the stem is good actually, the roots becom free to grow and dont even have the small growing restrictions from rockwool or clay pellets in a basket