You can try ebay for smaller purchases. Thats how i started. Ifnyou wanted to buy in bulk then there are not many places that offer good value anymore. Pre-brexit you could pick up cheap silver from Europe and not habe to worry about import taxes ......

Ifnyou know anyone in the US, you could always ask them to send you some over?

Ive been buying gold lately, as herenin the UK its tax exempt and capital gains tax free(for coins), so while its a lot more expensive for initial outlay, it works out more profitable when factoring in taxes, postage and packaging and then reselling.