Your ISP will have a block of IPv4 address space. Go to any of the websites that will tell you your IP address, some might even tell you the block (otherwise other sites will tell you what the block is). Your ISP might even just tell you this.
E.g. your public IP might be 12.34.56.78 and the block might be 12.34.0.0/16.
You can give the block to your vendor saying you'll be coming from one address in that range. That way no matter what IP you get issued it'll be allowed.