Short answer — try these places in Singapore (grouped by type). I’ve included examples/links so you can check current stock:

High‑street / chain (often stock polka‑dots or yellow printed dresses)

• Love, Bonito — Singapore label with multiple polka‑dot styles in its online/physical stores. (lovebonito.com)
• Zara (Singapore stores & site) — frequent polka‑dot and yellow dress drops. Check the “yellow” and “polka‑dot” dress sections. (zara.com)
• H&M Singapore — stocks yellow dresses regularly and seasonal polka‑dot pieces. (www2.hm.com)
• Mango / & Other Stories / Next / Alyssandra — carry polka‑dot and spot prints in their Singapore e‑shops and stores. (shop.mango.com)

Designer / premium stockists (for runway‑style yellow polka‑dot pieces)

• Mytheresa / Net‑A‑Porter / The Outnet — carry designer polka‑dot and occasional yellow polka‑dot dresses (e.g., Alessandra Rich and other labels). They ship to Singapore or have SG sites. (mytheresa.com)

Local online marketplaces / multi‑brand retailers

• ZALORA Singapore — big marketplace for many local and regional brands; good for searching “yellow polka dot” across labels. (zalora.sg)
• The Stage Walk / other local fast‑fashion e‑stores — sell affordable polka‑dot dresses across mall outlets in SG. (thestagewalk.com)

Vintage / one‑offs and indie boutiques

• Haji Lane boutiques & vintage shops — great for unique, retro polka‑dot finds and one‑off yellow pieces (walk the lane or check individual shop IGs). (visitkamponggelam.com.sg)
• Local vintage sellers (PicnicSunday, Etsy sellers shipping to SG) — good for true retro/yellow polka‑dot dresses. (picnicsunday.com)

Rental (for event looks without buying)

• Style Theory — Singapore clothing rental/subscription with designer and statement dresses available to rent for events. (Handy if you want a high‑impact yellow polka‑dot look just for one occasion.) (styletheory.co)

Quick shopping tips

• Search sites with exact keywords (“yellow polka dot dress”, “yellow spot print dress”) and filter by colour/print. (Works well on Zalora, Love, Bonito, Zara and H&M.) (zalora.sg)
• If you need a very specific runway/designer piece, check Mytheresa / Net‑A‑Porter / The Outnet for yellow polka‑dot designer dresses. (mytheresa.com)

If you tell me whether you want high‑street, designer, vintage or to rent, I can pull a few live product links (or search specific malls/areas) and check current availability.