As a professional costume conservator/mounter of long standing, of course my first response will be “It all depends”. <g> And possible solutions depend on a number of factors:
Where are you located – would it be possible to reach out to other regional heritage collection agencies that might have mannequins you can borrow?
The costume/dress are for female; for male; a mix of the two? Female body type mannequins are more common.
What culture and time period are the costume/dress from? Those from European/American/Western sources in the last 100 years and are in excellent physical condition will be easier to find appropriately sized/shaped mannequins than those that are not from Western sources, are older than about 100 years and are not in excellent physical condition currently.
Are there costume dress that are not tailored/cut and sewn to 3-D shape? These costume/dress that are not tailored/cut to shape (i.e. kimonos; thobes; etc.) may be able to be displayed on free-standing T-stands or tubes hanging freely or next to a wall.
What are the measurements/sizes of the costume/dress? The sizing and appropriate proportions of European/American/Western from 100 years have changed a lot over that time. You most likely need to be looking for mannequins that are nowadays sold for children and teenagers and not for adults.
Good luck!
Regards,
Margaret (Meg) Geiss-Mooney PA-AIC
Costume/Textile Conservator
1-707-763-8694
meg@textileconservator.com