As to scaling, First gear's main scaling is 1/34 and amongst others they also use 1/25. The latter is certainly the easiest to obtain accessories for. On EBAY there are a number of 1/24 & 1/25th items listed and also "G" Scale, which is supposed to be 1:22.5. However, there seems to be some confusion regarding the latter as I have seen it listed as 1/25 and 1/22.5 amongst others. Pola "G" and Woodland scenics are 1/22.5 as is Preiser "G" and Bachmann "G". Preiser figures are produced in a number of scales including 1/25 1/29 (2 1/2" or 60mm), and 1/32 is 2/14" or 534mm. Confused? so am I and I have ordered a catalogue, so check each item before you buy.
As to FG's 1/34 scale is a scale that offers nothing much in terms of accessories but 1/32 (Gauge I or British "1 Fine"), is close and this too offers a great range of items. As with all the scales, they are mostly orientated to railway layouts but depending on the choice of vehicles you collect, there are a number of suitable items to include in a diorama or just to put on your display shelves. Nought / O, or Zero scale is also quite common and equates to 1/43.
The following items were some I found on EBAY but try different search criteria as not all items are listed under one search name. For general information on modelling and painting techiques etc. try HOT WORLD CUSTOMS.
APPROX 1/25th SCALE - FROM BOXCAR BRUCE
5 GALLON CANS
DOGS & HYDRANT
DOGS & HYDRANT
APPROX 1/25th SCALE - FROM PSF HOBBIES
POLICE LINE TAPE
JOHN DEERE PUMPS
BOTH "G" 1/25th AND "O" 1/43rd - FROM RAIL PRESS
SOME OF THE OTHER ITEMS SPOTTED ON EBAY
( 1/32 scale many available)
FIREFIGHTERS AND KIT SUPPLY (unpainted)