In China, Hong Kong, Japan, etc., there are many buyers who want to buy more than 10k BTCs to find me to act as an intermediary.
But there are not many sellers on this side. Is there any way to find more real sellers?
Those people can simple go to many of the exchanges and just buy themselves, there is no need for a middleman for purchasing. Think of binance, bittrex, poloniex, coinbase, bitfinex, cryptoia, kraken and many more with the addition of localbitcoins as well.
If you want a trusted website to buy over 10k worth of bitcoins you can just use binance but be careful to buy slowly because if you purchase all of a sudden two problems will occur, first they may get suspicious about the activity level and second the more you buy the more you pump the price and will need to pay premium.