Frequency Generator - Amazon.com

my first frequency generator was a Heath HG -- these are pretty inexpensive on EBay -- one practical suggestion however is to disconnect the meter -- in Audio Amateur in the early 's there was a POOGE article on the IG18 or which suggested this to reduce the meter's effect on distortin numbers. In fact they had a little transistor amp which isolated the meter.

you will often find the Wavetek sweep generators on EBay quite inexpensively -- I have a 182 which cost me $40 -- not as low distortion as the heath, but it really helps when you are working with filters.

sometimes you will find that serendipitous point on EBay when a nice piece of HP equipment has not bidders -- I have an ovenized HPB which cost me ~$100 == the only problem is that it uses mini-WECO connectors which are a tad expensive.

some equipment comes with freq generators -- like the HP339 distortion analyzer -- this unit has a very high quality oscillator -- distortion around 0.002%

the HP wave analyzer usually goes for $50 - $100 and has a nice sweep oscillator -- it also has a very accurate audio voltmeter!

you really can't go wrong with the Krohn-Hite audio oscillators -- these are often pretty inexpensive and KH has an excellent reputation for quality.

things not to buy -- the old RCA and EICO tube based generators have pretty high distortion --