Start and Select Button(s)

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

The buttons were likely manufactured in Hong Kong or the Guangzhou region of China, as that is where many online retailers are sourced out of.

Hong Kong or the Guangzhou region of China.

The buttons were likely created in Hong Kong or the Guangzhou region of China.

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

Most of the mass online retailers ship through China Mail or Hong Kong Mail (https://www.aliexpress.com/item/1-Set-Rubber-Conductive-Buttons-For-Nintendo-For-Game-Boy-Classic-GBC-Silicone-Pads-Buttons-A/32787843170.html?spm=2114.01010208.3.2.teEKFC&ws_ab_test=searchweb0_0,searchweb201602_4_10065_10130_10068_433_434_10136_10137_10138_10060_10062_10141_10056_10140_10055_10054_10059_10099_129_10103_10102_10096_10052_10053_10050_10107_10142_10051_10106_10084_10083_10080_10082_10081_10110_10111_10112_10113_10114_10078_10079_10073_10070_10122_10123_10124,searchweb201603_6,afswitch_1,ppcSwitch_5,single_sort_0_default&btsid=132945c7-395b-440b-a423-0db7e1c55f0d&algo_expid=e5bf8e39-d365-4f36-a86c-6a63bddd1922-0&algo_pvid=e5bf8e39-d365-4f36-a86c-6a63bddd1922), so we can deduce that they were mass produced in Guangzhou or Hong Kong.