You can get selenase (selenium) and zincasol (zinc) on prescription from your GP if hairloss is an issue. It is more of an issue in bypass patients though due to malabsorption. If sleeve/band patients get hairloss it is generally due to rapid weight loss, like you get with cambridge diet etc, and will right itself soon enough.
I had hair thinning, but as I have masses of hair to start with I actually enjoyed it. Saved me a fortune in having it thinned at the hairdressers! Mine lasted about 6 months, then started to grow back. I had zinc and selenium prescribed, but didn't take them as I couldn't tolerate the taste. When it started to grow back I decided to get it all cut off though, as I had a lot of short wispy hairs which looked daft.