Yes, you can buy accessories for your SUP. It depends on the stand up paddle board you have purchased as different brands and designs have different hooks / magnets / attachments that come already incorporated into the SUP, which you can then add to.
Most SUPs come with a leash and a hook on the board where the leash can be attached to. This also goes for SUP seats. Some SUPs will come with rings or loops so you can purchase a SUP seat and add this to your board, so that you can double up as a SUP kayak.
If you want to add to your paddle board and it doesn't come with D-rings, you can purchase these and add them using PVC compatible glue.
You can add a SUP carrier, these are straps that loop round your board and attach to an over the shoulder strap, so you can carry your SUP with ease. Some SUPs will come with D rings to attach the strap directly to it.
You can add a cooler to keep your drinks cold. KULA make the perfect round coolers that attach to the Bote SUPs, and they even double up as a seat.
You can add a paddle holder to your SUP. These are handy especially if you are fishing and need free hands to deal with the fishing rod.
You can add coffee cups or drink bottles. Some SUPs come with magnets to hold cups, and some have attachments where you can attach a cup holder to the SUP.
You can add a fishing attachment that holds your fishing rods and a tackle bag or dry bag.
There is so much available to accessorize your SUP to make it unique to you, and more importantly, to suit your stand paddle board needs.