Zapier does not have one static range of IP addresses. Instead, Zapier uses Amazon (AWS)'s US-EAST-1 region, spinning up instances as needed. This means there could be millions of IPs to whitelist, which often becomes impractical.
You can find Amazon's list of current ranges (subject to change) here. We might be connecting from any of the us-east-1 ranges listed on this page: https://ip-ranges.amazonaws.com/ip-ranges.json.