A BMX stem is what connects the handlebars and forks to the frame.
To fix BMX handlebars, you might need to bend the handlebars back to their correct position. You might also need to tighten the bolt that anchors the handlebars to the bike. Fixes will vary depending on what is wrong with the bike.
Usually nothing more complicated than "BMX bars".
No I cant answer this question, how do i do it.
Yes, all handlebars have the same diameter out by the grips.
Tighten the stem
There are a few places where one can purchase a BMX cruiser. This includes going to a BMX bike store, as well as looking at cruisers online on sites such as eBay.
One can purchase BMX stickers online at websites such as DansComp, Zazzle, Lux BMX, and Supercross. One can also find them for discounted prices at Amazon or eBay.
If one were to purchase a freestyle BMX bike, one could go to a specialist bike shop. Alternatively, various specialist companies, such as Vital BMX offer a huge range of products.
you probably want to replace them. They can be pretty cheap to buy off the internet.