We live in mid-Michigan and the closest flea market is in Bay City. It's open every weekend from early spring to late fall and most years we've found excellent selection in April and May. I've bought a lot of glass there in the past 8 year and the biggest challenge was to be sure the dirt was really washable dirt and not haze or something permanent.
This year we've gone there three times and have bought very little. There has not been much glass and what there has been has been priced about what I can sell it for online or beat up - or both.
There's another big flea market twice a year a bit further away in Amish country and we went there today. This is a neat experience even if you don't find much to buy because it's in the country and many people there get around by horse and buggy. The ladies all wear warm-looking hats, dresses in plain fabric that go below their knees, black sturdy shoes and the guys wear hats and many have beards. Some of the Amish bring their jams and hand-crafted wood and quilts so this is not your ordinary flea market.
Sad to say, we got only a few pieces of glass. Someone had a Fostoria Vesper etched console bowl, which they had last year too, and this year I could not say no. And we got a couple etched candleholders and a few pieces of Federal Diana. Most of the glass there was chipped and overpriced, for example, $28 for an Iris iridescent ruffled bowl, or $35 for a chipped Cameo cake plate. But hey, one thing I do enjoy is buying glass. If it was in good condition and anywhere near reasonable - I bought it!