I'm wanting to seperate my bully platy with a net, rather than a seperate tank like I currently have, but all the nets i've seen are small (14cm across)... can you get deeper/longer ones anywhere?
Yes you can, I've a net 10inch by 8 inch and it's called Medium.
However I don't understand what you want to do. Are you saying your planning on catching your fish & have him live in the net? Sounds like a bad idea. If he's so much of a problem I would get rid of him & give him to a LFS.
I'm guessing he's a male & is bullying other males. Right?
The title suggests they're breeding, correct? You can buy big nets for this if you wish. Or alternatively you can get a breeding trap designed for them.
If it is a case of a problematic fish its best to put up, or get rid of it.