Two part closed cell spray foam is hands down the most effective air sealant we know of. If you just have rim joists, a crawl space, or another smallish space, this could be a fit. If you have a lot more than this, hire someone. This stuff is finicky, be sure it’s warm enough and read the directions. You’re unlikely to find it under $650-700. 200 board foot kits are about $300, some big boxes carry them.