How can I get a list of all symbols available via getSymbols in quantmod?

Try

library(quantmod)
values <- oanda.currencies
#> head(values)
#    oanda.df.1.length.oanda.df...2....1.
#USD                            US Dollar
#AFN                  Afghanistan Afghani
#ALL                         Albanian Lek
#DZD                       Algerian Dinar
#ADF                       Andorran Franc
#ADP                      Andorran Peseta

      

        
        
        
      

    

This gives a single column data frame containing a total of 191 currencies that are available from src="oanda"

.

Characters are stored as string names:

#> rownames(values)
#  [1] "USD" "AFN" "ALL" "DZD" "ADF" "ADP" "AOA" "AON" "ARS" "AMD" "AWG" "AUD" "ATS" "AZM" "AZN" "BSD" "BHD" "BDT"
# [19] "BBD" "BYR" "BEF" "BZD" "BMD" "BTN" "BOB" "BAM" "BWP" "BRL" "GBP" "BND" "BGN" "BIF" "XOF" "XAF" "XPF" "KHR"
# [37] "CAD" "CVE" "KYD" "CLP" "CNY" "COP" "KMF" "CDF" "CRC" "HRK" "CUC" "CUP" "CYP" "CZK" "DKK" "DJF" "DOP" "NLG"
# [55] "XEU" "XCD" "ECS" "EGP" "SVC" "EEK" "ETB" "EUR" "FKP" "FJD" "FIM" "FRF" "GMD" "GEL" "DEM" "GHC" "GHS" "GIP"
# [73] "XAU" "GRD" "GTQ" "GNF" "GYD" "HTG" "HNL" "HKD" "HUF" "ISK" "INR" "IDR" "IRR" "IQD" "IEP" "ILS" "ITL" "JMD"
# [91] "JPY" "JOD" "KZT" "KES" "KWD" "KGS" "LAK" "LVL" "LBP" "LSL" "LRD" "LYD" "LTL" "LUF" "MOP" "MKD" "MGA" "MGF"
#[109] "MWK" "MYR" "MVR" "MTL" "MRO" "MUR" "MXN" "MDL" "MNT" "MAD" "MZM" "MZN" "MMK" "ANG" "NAD" "NPR" "NZD" "NIO"
#[127] "NGN" "KPW" "NOK" "OMR" "PKR" "XPD" "PAB" "PGK" "PYG" "PEN" "PHP" "XPT" "PLN" "PTE" "QAR" "ROL" "RON" "RUB"
#[145] "RWF" "WST" "STD" "SAR" "RSD" "SCR" "SLL" "XAG" "SGD" "SKK" "SIT" "SBD" "SOS" "ZAR" "KRW" "ESP" "LKR" "SHP"
#[163] "SDD" "SDP" "SDG" "SRD" "SRG" "SZL" "SEK" "CHF" "SYP" "TWD" "TZS" "THB" "TOP" "TTD" "TND" "TRL" "TRY" "TMM"
#[181] "UGX" "UAH" "UYU" "AED" "VUV" "VEB" "VND" "YER" "YUN" "ZMK" "ZWD"