I've run into this also - the difficulty of finding standoffs in appropriate lengths, size and thread, and have done a lot of searching. One thing I've learned is that what I would call standoffs (lengths of tube or extruded hexagonal shaped material with internal thread) often go by other names, such as 'threaded spacers,' or 'sleeve nuts.'
I've found them at hardware stores (Ace, Lowe's, McLendon - here in Washington State, USA) in the fastener department, though the selection tends to be narrow and the prices perhaps a little high.
Most recently, I was looking for the right size of these to space a choke off of a Crack chassis plate, and hit paydirt on Amazon:
https://smile.amazon.com/gp/product/B00PKFL8II/ref=ppx_yo_dt_b_asin_title_o09_s00?ie=UTF8&psc=1Note that they don't call it a standoff in the item title, but it appears in the description. Iirc, Amazon had this size, but not necessarily the full range of lengths and thread sizes.
Hope this helps!