Try mo sa codashop, nagbenta sila ng steam credits, various ways to pay, pwede mo gamit load mo although mas mahal ata. Pag may app ka 2 legit apps that I know of sell steam credits.
The first one is shopee search mo steam wallet, diretso ka sa official shop nila. The other route is sa coins ph na app -> all services, tapos game credits. Mas easier siya kaysa shopee pero the only way you can cash in is either through 7/11, gcash, cebuana, or wire transfer from bpi/securitybank/unionbank (takes 24hours or less for it to get credited). Best option talaga is gcash kung meron ka.