I'd go to any local wrecker that has some old Falcons/Holdens/Valiants which had simple inertia belts that bolt in easily. Earlier models had the static 3 pointers as well. They are also available on Ebay brand new delivered for under $50 a pair.
Would it not be easier to buy a seat with the belt built in, like out of a minibus?
Buying just any belt will not work as the mounting angle of it is important in its opperation, thats a 3 point inertia belt anyhow.
Hope this helps.
Kezngaz.
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Commitment shows. Quality is what is done when no one is looking.
Looks like the Ebay ones may not be "legal" so I suggest you find someone on Gumtree who is privately wrecking a old car - there are a few up here in Toowoomba so should be heaps in the big smoke. Repco and SuperCheap used to sell new ones but not sure if they still do. Usually heaps for sale at swap meets.
If you are not familiar with Gumtree just PM me and I can quickly look up what's available around Redcliffe and PM you their phone number.
More than one way to skin a cat.