I recently wrote an article for another online source on this topic and will find it and email to you if you like.
In the interim, I suggested the following:
If unable to afford a sewing form [and I too had to get an XS when I got mine years ago], go smaller in your search.
You need to check out flea markets, garage sales and resale shops. If running across a large child size form, buy a roll of foam and pad it to equal and duplicate your body measurements. It's often easier with adjustable forms to stuff them first with batting, when wrap with tightly woven fabric under the foam, until achieving your measurements.
Remember to measure carefully, keep a log of your measuremenets and date it and update it regularly soi as not to need to alter garments after making them on the form. Threads has a good online article on measurements that are critical in the event you have no sloper.
If you want my full article reprint, I have a cc I can email to you.
Good luck & sew forth,lou-
<lou-dezyzn@sbcglobal.net>