You can use a smartphone for streaming - you just have to dowload an app that turns it into an IP cam. Search for IP cam on Play store - it's free IIRC
Alternatively you can get one of those cheapo wifi routers like MR3020
or TL-WR702N (might be the same model actually but the company sometimes uses different model names for different markets), flash it with openWRT and add some drivers and do some configuring to get them to stream video. There are literally tons of tutorials on doing this on the internet so just use google and you'll find what you need
Ohh you might as well just use hardware used for FPV in RC models, but that's a bit pricier.