can i get a new suspension kit for a older mongoose XR-75 I've searched but I can't find anything online. Is there a store I can go to or something?
This is a bso. You won't find replacement parts that aren't maintenance items (brakes, tires, tubes, freewheels, etc.), since they will exceed the cost of the bike (particularly big things, like suspension or wheels). The suspensions on these bikes are mainly for aesthetics, not for actually taking hits in mountain biking (full suspension bikes are jolly expensive; front suspension only bikes are not that much -- few hundred bucks to start for decent basics, cheaper on used market). 

You're better off getting a new bike if you need suspension, or living with the suspension as is.