When we first build the big Aviary I have now ~40x50ft., we bought some clumping bamboo for the pen. Bamboo can be pricey as the large ones were $100.00 or something like that. We did not want to spend much so we asked for a very small pot. We got a pot of two or three very thin, yellowed, short canes (the canes were supposed to be green). The peafowl didn't eat the leaves or shoots, and now it has matured very very nicely. You can certainly start with something small when it comes to bamboo and it won't be long before it is big (I didn't even do all the things the bamboo seller told me to do to the bamboo and it is growing really well, although I do put peafowl poop around it occasionally).
This year my peafowl pecked at a few of the new shoots, but I would say there are around 20 new shoots so they definitely have not eaten on nearly all of them. I read that in the fall or winter you should cut any of the canes that are getting old or don't look so pretty. There are some center canes that I think I should trim because they are looking yellow and they are small.
Sometimes you have to put a fence around the plant until it is large enough. A lot of people do that to help keep plants thriving in the pen. I planted a cyad in the aviary that I got for cheep on the side of the road. Some landscapers had to dig up all of someone's cyads and then they didn't know what to do with them so they decided to sell them. We put it in the ground and all the leaves died off of it. I thought it was dead until this year when it sprouted leaves. Cyads have very tough leaves, but the new leaves are very very tender, which was irresistible to my peafowl. Sooo I got lucky and found some old screening and wrapped it around the top of the cyad to prevent my birds from picking at the new leaves. It has worked very nicely.
If you are not sure about spending the money on more plants, go into the woods and pick out a nice tree or bush and dig it up. Look for one that is taller than a peafowl so that they cannot eat all of the leaves on it.
Oh one more thing, some evergreen trees have popped up in my pen, and I noticed the peafowl never eat on those. They are kinda Christmas tree-like. I don't know exactly what kind of tree they are.