I just went onto my TD Ameritrade account and it seems like if I wanted to, I am able to buy it- they gave me the option to reinvest in this fund, etc..
But again, personally, I wouldn't buy VTSMX through TD Ameritrade, because they charge their owns fees for purchasing this mutual fund through them, where as the ETF version is commission free. If you insist on buying into the mutual fund, and you have the option, doing it directly through Vanguard's website is probably the best bet.