Where can I get replacement foam spacers and seals.

It would be helpful to be more specific on what foam seal or insulator you need to replace.

Given the systems age new parts are hard to come by, and as Apple no longer sells parts to their AASP's and most have long since stopped servicing this series the parts they had are likely sold off or have gone into the bin. There are still keyboards available and batteries which are knockoff's

That leaves you with conscientious recyclers like TheBookYard in the UK or what iFixit is able to get which are my top goto's. There are others that strip down systems to resell the discrete parts Seals and insulation sheets are just not often available. Here's the TheBookYard parts listing for your system you could try dropping them a note asking if they have what you need.

While not easy I often use a sheet foam or even felt, or even graphite impregnated plastic of the needed thickness to cut out to the required shape to replace the damaged part. I've even had a friend with a 3D printer create replacement internal plastic parts for me and we even tried to make a mold to cast Silicon rubber for a special project.

You generally can’t buy official foam spacer/seal kits for the MacBook Pro 2015, but many DIYers cut replacements from thin adhesive craft foam or thermal gasket sheets since Apple doesn’t sell them separately.