Hi agnelors,
A few options: Stanford Research's SR830 is sort of the standard for a digital lockin. Max frequency of a bit over 100 kHz (102, I think), runs about $5K new. It's been on the market for a couiple of decades now, so picking up a reconditioned one is easy enough and would save some money.
Signal Recovery has several options; I've had good experiences with their 7230 (120 kHz default; upgradeable to 250 kHz). Without the 250 kHz upgrade, it lists for I think about $4K. No front-panel display or controls, so everything is either through a web-browser front end or USB from LabView (there's a driver available). The 7265 adds displays/controls and includes the 250 kHz option; I think it's about $5K, but I'm not sure. The 7280 mentioned above is the wideband option; it goes as high as 2 MHz; I've never had occasion to ask for pricing, but I'm sure it's more.
The gold-plated solution is an outfit called Zurich Instruments. Their _low end_ model goes up to 50 MHz. Price is commensurate (I asked at one point out of morbid curiousity; it was multiple tens of thousands of dollars).
There's also a few implementations of lockins in software (including in LabView). If you want to stick with your current DAQ setup, that's what I'd recommend for getting the phase information out of the raw timeseries data. That's sort of a money vs. time question; the commercial boxes will do the job basically with little to no setup time, but they're going to be more expensive than buying (or using an already in-hand) digitzer and spending the time needed to develop the necessary software.
