Baird and co are good.
https://www.bairdmint.com/
also
Weighton coin wonders
https://weightoncoin.co.uk/
if you know what you are buying there is usually a good selection on ebay. Look for sellers with 100% feedback who have sold lots of precious metals before.
If you are near london the coin dealer in charing cross road is very good.
Alternatively consider gold or silver ETFs available on most share dealing sites or buying shares in mining companies.
There is also a bitcoin type thing available that is backed by gold. I don't know much about that but do a search on here because I think someone posted a referal link at some point for free gold.
Historically, silver is much lower priced compared to gold at the moment so you may want to think about weighting the purchase towards silver.