There are many items the Fremen need that are not able to be made out of spice- boots, thumpers, material for windtraps, mining equipment (for the occasion when they are digging new shelters in a rockcliff).
Even if they could locate the ore for making the above- the machinery is a bit too complex- unless there is a Ix like tribe out there in the desert with welding equipment and electrical generators.
Plain and simple- they were dependent on the outside for the height of their population..... I would love to know how they got their materials for radical horticulture adapted to the extremes of Dune from Spice, Sand, Spice, Twigs, and Shit. They are a society under continuous attrition from nature and one another, and furthermore have CHOAM and the Harks to worry about..... you add their terraforming quest to the mix..... they are some big spenders.
This being said- they appear to put much, much more emphasis on the group that can move, that can fight, that can reproduce- they lived a bit like Russian nihilist, with high emphasis on what was useful- and in a manner very similar to Juche- severe self reliance. It seems the only wealth that mattered to the individual was that you could carry on the back of a worm.... and what was property didn't extend much farther than that.... save in water banking. They none the less are astounding in the degree of redundency they built up- Stilgar's desire to retreat deeper and deeper into the desert- suggests advanced preparations in supplies and secondary locations...... more water traps, more stockpiles of thumpers- globelights, perhaps even more clothing- such as boots.
I have my doubts the Stilsuits are made from all desert material.... of all of Spice's properties, water resistance and holding capacity isn't amongst them..... the stuff dilutes easily in water. I suspect all the parts none the less are found on planet- it would be all to easy to kill off the fremen by blockading a essential supply- and all too easy to convince the spice smugglers NOT to supply the Fremen with the parts they need to maintain it- the Fremen were the Smugglers main competitors and threat- it wouldn't take that big of a bribe to convince them not to help them.... if one was even needed. It's the only reason why I can figure the Fremen keep coming back to the city in the first place- they need stuff- silly simple stuff- and every planetary government allowed it because it was a safe and simple source of taxations, and seemed such a small triffling in allowing, and yet impossible to suppress..... off planet imports though are easy to repress.
Gives you a hint at what the cities on arrakis) were manufacturing. Weld shops, leather workers, looms, stilsuit manufactures (for the city folk), distrubutors of raw materials for suits, material for upkeeping windtraps (deeply suspect the fremen were not dumb enough to order massive amounts of sheet metal, arrive in mass in the city, and be seen by military/police authorities walking out into the deep desert with it..... that's one of those Norm McDonald 'Note to Self' moments when a detective stops and thinks WTF is going on, gotta check up on this one. (I also can't imagine how many tiedowns and bungie cords it takes to carry the sheet metal on the back of a worm- I can see them under a tarp- and some hillbilly holding it and getting crushed or loosing a limb. This being said- smaller stuff like screws and handtools are going to be gotten legitimately in market and not via smuggling)