Making a vegan potato-puree like course without whole potatoes
You're vague about what's available at a reasonable price in your area, but there are plenty of things that are used to make vegetable mash:


Root vegetables.  (Turnip, parsnip, yams, sweet potatoes, rutabaga/swede, carrots)
Winter squash.  (Pumpkin, butternut squash, acorn squash, etc.)
Cauliflower


None are necessarily a direct replacement, as some are more strongly flavored, some have more moisture (but you can bake/roast them to help dry them out before mashing), and some are more fibrous (cut up spaghetti squash and sweet potatoes across the fibers before cooking, to reduce the length of the strands).

If you really want to use potato, then look for 'potato flakes' not 'potato starch', but make sure to follow the package instructions, as just like real potatoes, they can be gluey if you're not careful.  (I suspect it's that they need some fat ... I usually use olive oil, although you may want to go with something more neutral in flavor).

They can also be reconstituted and mixed with other mashed vegetables.