Metal bolt sleds are quite expensive, ranging from $60-$80, unless you have the materials to fabricate your own, which even then would require a lot of precise measurements. I recommend you just buy another plastic LS bolt sled and use epoxy to attach an aluminum (.032 thickness is good) or thin steel bolt sled cutout. This only cost me about $5-$8 to do and I have plenty of extra aluminum to reinforce other parts. Also, since it requires you two hands to prime, you might want to make/buy a shotgun foregrip for ease of priming and better distributed force on the bolt sled.
Metal bolt sleds and full upgrade kits can be bought from eBay for about 30$-40$. Not sure where you found overpriced parts...
Not sure if I should link the site, but you can search eBay for nerf mod kits and look for the professional looking ones with boxes and parts listed.