Any recommendation for thistle on Amazon? I don't see anything particularly cheap.
Amazon is terrible for thistle, no idea why.
I've found that the best way to get them is in-store.
Walmart has a generic that is my favorite (and also the cheapest at ~$1.30/lb)) - but it's available in-store only. I've never seen my local Walmart (Monsey) out of stock, even in winter when they don't have a lot of birdseed.
Another option is
this from Tractor Supply Co., but only makes sense for store pickup as shipping is quite high. I haven't tried this particular seed, but I've been very happy with other seed from this brand. It's a touch more expensive than Walmart (~$1.40/lb) but might make sense depending on your location.
Finally, when looking for thistle, also check for Nyjer seed - same thing, different name.