Are 'Do it yourself' assembling kits for mobiles/ smartphones available in market?

There are many companies working in this space. And they seem to be keeping both the basic kits as well as the complex kits.

Make sure to check out the GSM based kits in these sites. You can also check out some of the other communication based DIY kits. And connecting the dots you'd get smartphone specific assembly in general. Or you can check the complete smartphone kits.

1. The magazine named "Electronics for you" is having such kits on sale. Though often the availability of the kit in such case is a bit debated. Also the shop has some products taken off the shelf randomly. So you may or may not find the kits of different type all the time.

You can check the mobile DIY kits here: https://grab.electronicsforu.com/

2. Kits and Spares is another website where you can find the DIY kits. GSM based kits are available over there. So that's something you can check out. https://www.kitsnspares.com/77/22/do-it-yourself/gsm-based-kits

The pricing for the mobile and smartphone kits start from 4000 onwards to anywhere upto 20000. The price may go up and down depending on what type of kit you want.

There is this Seed Studio who has recently launched a campaign to get funding for its Kickstarter project. They named it 'RePhone' which is a modular smartphone kit with which you can start your DIY trial from a scratch. The kit packs in it a core module, with an ultra-small System-on-Chip (SoC) to be used for every kind of communication protocols, like GPRS, GSM, and Bluetooth. The kit gives you a 1.54-inch touchscreen and a pack of the battery along with a smartphone case. They will give you a foldable case made of Kraft paper to organize and keep the parts.
you can get the kit here: https://www.seeedstudio.com/
Price: $45