The base of your Georgian outfit is a cotton or linen shift/chemise, next comes the stays, then underpinnings (normally panniers, bum roll or rump), then under-petticoat, next the top petticoat (the skirt), and then you have a choice of caraco (short jacket), or a full length gown: robe a l’Anglaise, robe a la Française, robe a la Polonaise. Your outfit will be complete with a wig, cap and hat (normally a bergère or tricorn).

Stays (not the Victorian tight laced corset) are important, as a lot of the weight of Georgian clothing is tied around your waist, so you don’t have any discomfort, stays to tie the clothing around are recommended.
Cheep-ish example stays from eBay
More expensive Redthread stays
More expensive Nemuro stays
What is important is that you buy stays with tabs at the bottom, so they do not dig into you.

Underpinnings
Example bum roll from eBay
Example rump from eBay
Example panniers from eBay

If you don’t want to go down the more historically accurate route, and/or don’t want to wear stays and/or don’t want to sew, I would suggest a cheap dress from eBay or Amazon. These are suitable for Georgettes, but may not be suitable for all Georgian events in the UK, and most certainly not for reenactment
Light in the Box purple floral dress
Light in the Box green floral dress
Light on the Box cream floral dress
Light on the Box white floral dress
Light on the Box pink floral dress
Gold Amazon dress
Gold and Black Amazon dress
Tartan Amazon dress
eBay robe a l’Anglaise
eBay purple robe a la Française
eBay pink robe a la Française