You can buy a white halter in the general store, when you deliver it you get back a little more than you spend buying it thankfully.

The dandelion western pad thing is a dye (dandelion is a seasonal spring dye you can find on hoofbeat plains) then you would take a white western pad from the store and dye it to complete that one. 

(you might be able to find tack in the scrapyard sometimes, I'm a lot less familiar with that, but it definitely hasn't happened to me yet… though I don't try too often lol)

Thanks, I didn't know I had to buy it!

I am still confused with the dying though - while I don't have that specific dye, I do have a bunch of other ones, and I can't figure out how I would use it on the western saddle blankets. If I go to the ‘dying’ tab, the only options I have for ‘item 1’ are 4 different white flowers - I can't scroll down, so I assume there isn't any option for tack as I can scroll without issues on the other boxes. I can access my tack through the ‘encrust’ tab, however that one doesn't let me select any dyes for item 2, only jewels…

Do you have a white saddle pad in your inventory? If you have one then the option should show up at that point  :)

I usually skip any quests that are going to cost me that much, but I dye things for my horses sometimes and it shows up once I have the white halter/polo/saddle pad available. 

Ah, I'm an idiot - got white and /brown/ instead of just white! It's working now, thank you for the help!!

No problem!