Hi Leanne, and welcome to the VFG forums.
Plus size vintage clothing is harder for sellers to come by, for a variety of reasons. Some of it is that humans in general are taller and heavier than we were 100 years ago. Some of the ways women's clothing survived to eventually become "vintage" is that women tended to preserve the special fashions of their youth, while the clothing they "grew into" after starting families was more likely to be worn and worn until it couldn't be worn anymore. Another factor - except for very special occasions, I am not willing to strap myself into the undergarments that past generations of women took for granted as the price to be paid for shaping their body to the requirements of the clothing.
Plus size vintage is out there, but there is less of it and it is likely to be more expensive because of its scarcity.
Have you tried Etsy? Good search terms are XL, Plus, or Queen. I would also add VFG as one of your search terms so Etsy is more likely to show you items from sellers who are VFG members. VFG members adhere to a Code of Practice that includes offering complete measurements.
Last thing - pay attention to measurements, and don't hesitate to contact the seller if needed to ask for more measurements. Large and XL are somewhat subjective, but the numbers will tell the story.