My own family were farmers and labourers in and around Cockerham which is a few miles north of Garstang and it is still all dairy farming with some sheep on the marsh areas and fells.
Ditto. We both have Lamb ancestors. One of mine married at Garstang.
My book is "Garstang and District: Historical selections" by John Askew, published 2012. Chapter titles include: A year in old Garstang; Market; The poor; Agricultural and Horticultural shows; Claughton. A chapter "Principal buildings" includes the corn mill, corn exchange and tithe barn. An advert for the soon to be opened Garstang Corn Market appeared in "The Lancaster Gazette" 21st July 1808. The same newspaper reported on the success of the market in September 1808. (I've eaten lunch at the tithe barn by the canal and bought cheese at the market in the old corn exchange building.)
Sources of information in the book which may be relevant are:
various documents at Lancashire Archives;
local newspapers, mainly "The Lancaster Gazette" and "The Preston Chronicle & Lancashire Advertiser":
"The New Lancashire Gazetteer and Topographical Dictionary" by S. Clarke (1830)
+ some books by travellers who sojourned at Garstang en route north or south -
A. Hewitson "Northward" (1900)
William Hutton "The History of the Roman Wall" (1802)
Thomas Pennant "A Tour of Scotland and a Voyage to the Hebrides" (1772)
Pennant remarked on the fine cattle and "abundance of potatoes". He said potatoes were sent to other places in England, Scotland and Ireland. (That strikes me as sending coals to Newcastle.)
Garstang Agricultural Society was formed in 1809 and held 1st show 1813. Reports in Lancaster Gazette. Mention of sheep and long-horned cattle. The neighbourhood of Garstang was well-known for this cattle breed.
Several fairs were held in Garstang in 19th century.
Candlemas Fair (February) was hiring time for servants.
Spring Fair in April began 1830s. Reports in "Lancaster Gazette" and "Preston Chronicle".
Cattle, wool, sheep and lambs were sold at Peterstide Fair in July. (Feast-day of St. Peter & Paul is at the end of June but the calendar change in England in mid-18thC caused the Fair to be held in July from then on.) Thomas Hutton (author of "The History of the Roman Wall", above) was in Garstang for Peterstide Fair 1801. The local country lasses made a BIG impression on him. They arrived in town in their best attire with their skirts tucked up to keep them out of the mud. He described them as ".. large as troopers ... exhibited limbs of gigantic size, well adapted for working, running or kicking ..." (I suspect he was confusing them with horses.

) He noted the men were similarly well-built. (My GGF whose ancestors were from the area was "well-built".)
Martinmas Fair in November marked the end of the farming year. This was a major fair. Sales of cattle and horses attracted buyers and dealers from all over Britain. An estimated 3000 cattle were at the fair in 1805. It was also a hiring fair for farm servants.