Short answer: you can buy yellow polka‑dot racewear from both Australian racewear brands/retailers and international shops that carry the trend. Below are reliable places to look (with quick notes and links to current product pages I found):

Where to buy — dresses & ready‑to‑wear

• Gorman (Australian label known for playful polka‑dot prints) — ships internationally; good for quirky, race‑appropriate day dresses. (gormanshop.com.au)
• Zimmermann (Australian designer — higher‑end, often makes yellow polka‑dot styles; available new and via resale sites). (lyst.com)
• Major Australian department & fashion retailers (Myer, David Jones, The Iconic) — large selections of yellow dresses and seasonal racewear. (Examples on Myer show many yellow/mustard pieces.) (myer.com.au)
• Global/high‑street sites that ship to the U.S.: ASOS (has butter‑yellow polka‑dot styles), Lyst (aggregates designer + resale listings). Good for fast, affordable options. (asos.com)
• Etsy & vintage marketplaces — excellent if you want vintage yellow polka‑dot dresses or unique independent‑label pieces. (etsy.com)

Where to buy headwear (fascinators / hats) for racing events

• Australian milliners and racewear hat shops (Ford Millinery, Fillies Collection, Morgan & Taylor, Fascinators Online, etc.) — they stock bright yellow pieces and ship internationally or take custom orders. These are the go‑to for race events. (fordmillinery.com.au)

Why yellow polka dots?

• Fashion coverage and race‑season commentary note a return to lemon/butter yellow and polka dots for the Australian spring racing season — explains why you’re seeing this look at race events. (dailytelegraph.com.au)

Quick tips for ordering from the U.S.

• Check international shipping & returns (some AU stores charge duties/import fees or have specific international rates). Gorman and many AU retailers list international shipping options. (gormanshop.com.au)
• If you want a hat/fascinator, buy earlier — race headpieces often sell out in the lead up to Melbourne Cup/Spring Carnival. Milliners also offer bespoke/custom options if you want a perfect match. (fordmillinery.com.au)
• For budget or same‑day needs, ASOS/ASOS Marketplace, Etsy or local US shops (Anthropologie, ModCloth, vintage stores) will have polka‑dot yellow options with faster shipping. (asos.com)

If you want, I can:

• Pull current in‑stock yellow polka‑dot dresses or fascinators in a specific size/budget and show shipping times to the U.S. (I already checked the retailers above and can run a targeted search).