I had good results on ebay by only buying from sellers who measured their clothes and listed key dimensions. Only one seller messed up and refunded my money on a pair of jeans.
Kohls lets you stack discounts. I got a 30% off coupon and used it to buy tops already on deep discount.

Pay attention to house brands, sizing is often consistent with a particular brand. Sometimes you can use this to buy something on ebay.
Finally, you can use your thrift store purchases as a guide for the brands to look for. I found Carole Hochman via thrift store purchase of a PJ top. In fact, I'm wearing one of her XS robes right now!