Unfortunately, a succession of US legislation and regulation (which America imposes on the rest of the world), has made dealing with anyone who is a "US person'" expensive, problematic and, quite frankly, risky from a regulatory perspective. Many brokers and banks outside the US have, understandably, decided that it's not worth taking on US clients.
Try Interactive Broker:
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I have no personal experience with them (so not a recommendation) but it would appear that they accept US persons (presumably because they are a US brokerage).