Aquabid and eBay occasionally have good breeder strains. Twin Cities Guppies is a local favorite of mine although they're low on stock at the moment. Guppy Train is a very popular Thai import seller but their guppies are expensive ($90 for a trio). There are some sites that specialize in certain strains too like Michael's Fish Room (he has a YouTube Channel), Live Fins, etc.
Just be aware that imported guppies tend to be more fragile and don't live as long; an F1 generation born in the US on the other hand will have a normal life span and be less fragile. The reason the imports don't tend to do well is because they're often raised in salt water - it's much cheaper to produce guppies in high volume using salt water in most parts of Asia.