Traxxas is nothing if not efficient with their parts use. The stock shocks for the Stampede VXL and the Slash 4x4 VXL are part numbers 3760a and 3762a for front and rear. Unless there's a weird exception, I think every 1/10 scale vehicle uses that same combination.
This is what you need. You might want different springs for a Stampede though, I hear the progressive Slash springs kind of suck. You can buy Traxxas springs or springs for the Losi Ten shocks also work. I got a set that had 4 different springs rates because I wasn't sure what I needed. Rear. Front.
Here are the parts on Traxxas' site. https://traxxas.com/search?keyword=7461
It'll cost more to get them and the springs separately as opposed to the eBay route, but you get to choose your color.
Long for the front, xx-long for the rear.
Integy has no reason to change their ways, it's flashy enough to be appealing and look decent to people who haven't heard otherwise. Same thing you learned about the cheaper off brand stuff. Price doesn't always correlate to quality, but cheap stuff on eBay/Amazon is usually cheap for a reason. And a fact of RC is that Integy will always be a bad bet 😂
Good luck with the GT-Rs, I think you'll be happy with them.
Honestly the best place is from one of the chop shops eBay. They buy slash ultimates and then part them out for big profit. But it's still a cost effective way to get a full set of shocks and springs. There are a number of listings right now.
If you search for Losi TEN springs, they sell a set of 4 different spring rates. Front and rear.
https://www.amazon.com/dp/B0059D3W9O?ref=ppx_pop_mob_ap_share
https://www.amazon.com/dp/B0059D3X5W?ref=ppx_pop_mob_ap_share