uhh, 500k USD is around 1.7k BTC these days, which not look so high, as it looks in fiat:)

possible ways

1* go to localbitcoins.com, initiate contact with biggest sellers in your region (Europe, US) and start negotiate terms / ask them for offer.
2* initiate contact with some huge Chinese mining company/visit China personally and buy it directly from miners
3* create multiple bank accounts, transfer fiat to multiple exchanges and start buying

problem with point 3 is, that gov. may block your funds at it will be visible. So imho best and feasible way is just start slowly collecting locally..anyway, it will be risky and time consuming for sure, 500k USD is really lot of money, in my country you can had luxury car and amazing big house with garden everywhere you want:))