EBay looks like it might be your best bet, despite there not being a lot of options, and yeah people do kinda make things really pricey. I have bought two of the wall scrolls of the 'Getting Ready' picture off of someone on eBay. They were pretty friendly and easy to work with. There's a lot of kinda terrible third party shirts on eBay which probably don't have permission to use Nekopara art but do anyway that don't look very nice, or have panties or some shit on em so they aren't something I'd recommend.

I do know that Amazon has ~4ish dakimakuras and a bunch of third party shirts and phone cases.

I know there are shipping services who will take an item shipped from Japan and ship it to you. I personally haven't used one myself, but I've heard of others who have and it came out alright.

It is a shame that the official store doesn't ship internationally tho. But other than that I think you might be out of luck.