The product which comes up most often as a recommended sealer for travertine surfaces and is considered to be the best in the industry is the Miracle Sealants 511 Impregnator Sealer. Miracle Sealants backs up the product by attaching a 20-year guarantee of satisfaction.
It is important to know that travertine will etch easily, but sealing has nothing to do with and won't prevent etching. Etching is the dulling in surface sheen and change in texture of the stone surface which is caused by acidic liquids found in foods and some cleaners. Sealing the stone will nevertheless prevent the absorption of other liquids into the stone.