A and B Buttons

Unknown material(s), manufacturer and/or supplier. Probably made of injection molded silicone rubber

Most of the replacement retailers are based out of Hong Kong and the Guangzhou province of China (the industrial district). Because there are no specific markings on the plastic pieces, we can not trace them back much further than this general region. (Hong Kong and the Guangzhou province are fairly close together.)

Hong Kong or the Guangzhou province of China.

These buttons were assembled in a factory in either Hong Kong or the Guangzhou province of China before being assembled into the Nintendo GBA.

The most definitive sources I have are online stores--large stores like Amazon and eBay, as well as smaller independent suppliers: you can buy "brand new" replacement Game Boy Advance buttons made of silicone rubber. Judging by the images, they appear to be exactly the same as the buttons on Leni's GBA.

For example, here's a complete set being sold on eBay: http://www.ebay.com/itm/Set-of-Replacement-Rubber-Conductive-Pads-for-Nintendo-Game-Boy-Advance-buttons-/332151670640?hash=item4d55c84770%3Ag%3A~6IAAOSwsW9YxCqz

And another from GameboyShack: https://www.gameboyshack.co.uk/product-page/gameboy-advance-replacement-silicone-rubber-pad-set

Mass online retail outlets sell the buttons in bulk from Hong Kong and Guangzhou, China. (https://www.aliexpress.com/wholesale?spm=2114.01010208.8.4.4nUEO0&initiative_id=QRW_20170316231230&SearchText=rubber+parts+game+boy&productId=32744322717 & http://www.made-in-china.com/manufacturers/gba.html)