The steam workshop has many, Then just google rf2 fan sites.(well maybe not that, but look for rF2 devoted pages and you'll find several locations to download 3rd party tracks.
Like it was already mentioned, the Steam Workshop is a great place to find tracks and all sort of official or third-party content. Some tracks though are only available here.
I have begun to form this "Worth to have content list" tracks 3PA_Bathurst_2014 3PA_Matsusaka_2015 AtlantaMP_2014 Barber_Motorsports_Park_2015 Botniaring_2017 Brianza_1966 Bridgehampton Castle_Combe_Circuit CRYSTAL PALACE 1969 Dubai_Autodrome ISI_Belgium_1966 Le Mans 91-96 Virtua_LM LimeRockPark_2012 Longford_1967 Miseluk (paid mod) MonteCarlo_1966 Mores_2013 Mosport (CTMP) Norisring Oulton_Park_Circuit_2015 RoadAtlanta2017 sm_Macau_v1.0 sm_Ring_Knutstorp_v1.0 T78_Hockenheimring Thruxton Watkins_Glen_International Zandvoort_2017 Sebring (DLC) cars AC_427SC_1967 AL_HQ_Monaro_v096 AL_LX_Torana_v096 ASR formula mods Audi_90_Gto BAC_MONO BMW_E34_M5 Brabham_1966 Brabham_BT44_1975 Camaro_2012 Caterham7_14K_Classic ER_Flat6_Series_rF2 FLAT12_917k_1971 Ford GT MkIV FormulaRenault_35_2014 Historic Challenge_EVE_1968 Historic Challenge_spark_1968 Howston_DISS_1974 Howston_G4_1968 Howston_G6_1968 ISI_Chevrolet_Corvette_ZR1_2012 ISI_Honda_NSX_1991 Honda Civic ISI_Renault_Megane_Trophy_2013 ISI_SkipBarber_2013 Lola_T280 March_M761_1976 McLaren M4B McLaren_M23_1975 McLaren_MP4_8_1993 McLaren_MP4_13_1998 Mercedes_190_EVO2_DTM Nissan_GT500_2013 Panoz_AIV_Roadster_1999 PM3DM_PrimeraBTCC Radical_SR3-RSX_2017 Renault_MeganeTrophyII_2013 SCES_Cobra_FIA_289 SpecMiata (search for link in this very forum, I have got broken physics from other place) SRW Ford Sierra Cosworth RS500 STK_Ferrari_Challenge2011 USF2000_2016
