Source For Loose Sci-Fi Weapons?

Lego! other makers do all kinds of generic SF weaponry and also 'true' Star Wars stuff. Some is too big, but it depends on the actual size of the figure. I use them extensively with SW figures for example, and also Tehnolog...indeed with any SF range I am modelling

www.firestartoys.com/Shop/Minifigure-Weapons.html

Warhammer 40K is 28mm but the oversize weapons mean you can use a lot of them, and they can all be found on eBay as loose spare parts.

www.ebay.com/b/Warhammer-40k-Heavy-Weapons/183473/bn_7023360857

www.ebay.com/sch/i.html?LH_CAds=&_ex_kw=&_fpos=&_fspt=1&_mPrRngCbx=1&_nkw=40k+weapon+bits&_sacat=&_sadis=&_sop=12&_udhi=&_udlo=&_fosrp=1

Same applies to makers of 40K-compatible weaponry - much is way over 28mm scale, especially anything listed as a 'heavy weapon' - they tend to be just right for 54mm.

spikeybits.com/2018/04/best-alternatives-to-gw-miniatures-revealed.html

thanks for those links Mike!