I’m thinking Tesco or Big C as well, but *maybe* CM Plastic will have something along these lines.
I can’t specifically remember seeing them there, but the place is so eclectic that you might just luck out.
It might be worth calling ahead before you go, ‘cause parking is a nightmare .
Oh, one other thought: Kamtien (sp?) Market might be worth a browse. All the pet fish supplies there...maybe it will yield you a pool.
Good luck!