Allowed to try it on in the nine days?
Try it or buy it? If the sale will not be available after Tisha B'Av or in the foreseeable future and it's a considerable saving then you can buy it and wait with wearing it until after the 9 days. You can try it on before buying it to make sure it's the right size under the above conditions. To try it for an alteration is ossur.
best to ask your LOR or look it up.