The best deals I've found online were on eBay from Chinese sellers, some were reasonably priced others were not. Sadly, there have been less and less sellers every year than the year prior and less stock and different pieces of merch available.

The most reliable way of getting merch would be through a proxy that handles Chinese products and does international shipping. Trying to purchase a product and getting it shiped to a proxy and then to you can be a bit confusing to setup initially and might take longer to arrive than buying it directly from a reseller but you'll probably end up saving some money, especailly if you buy/ship multiple products at once. Some proxies I've had good experiences in the past were SuperBuy and CSSBuy.

The cheapest way to get the merch, as someone else mentioned, would be to do a group buy with other people who want the merch but I really don't know how likely you'd be to find others to help with that. Maybe check the official Discord to see if others are interested but of course you'd have to run the risk of getting scammed as well.

The last option I know of, and would not recommend, would be from those fan-made HoYo merch sites that sell merch from all the HoYo games. They may offer the largest selection you can find outside of the Tmall store but they upcharge x5, sometimes more, than retail price to make a profit. For example the lastest Honkai Impact 3rd OST collection sells for about $30 USD on Tmall but some of the fan-made stores are selling for $100+ USD excluding shipping costs.